[gdb/symtab] Fix skipping of import of C++ CU

Tom Tromey observed that when changing the language in
gdb.dwarf2/imported-unit-bp.exp from c to c++, the test failed.

This is due to this code in process_imported_unit_die:
...
      /* We're importing a C++ compilation unit with tag DW_TAG_compile_unit
         into another compilation unit, at root level.  Regard this as a hint,
         and ignore it.  */
      if (die->parent && die->parent->parent == NULL
          && per_cu->unit_type == DW_UT_compile
          && per_cu->lang == language_cplus)
        return;
...
which should have a partial symtabs counterpart.

Add the missing counterpart in process_psymtab_comp_unit.

Tested on x86_64-linux (openSUSE Leap 15.2), no regressions for config:
- using default gcc version 7.5.0
  (with 5 unexpected FAILs)
- gcc 10.3.0 and target board
  unix/-flto/-O0/-flto-partition=none/-ffat-lto-objects
  (with 1000 unexpected FAILs)

gdb/ChangeLog:

2021-07-06  Tom de Vries  <tdevries@suse.de>

	* dwarf2/read.c (scan_partial_symbols): Skip top-level imports of
	c++ CU.
	* testsuite/gdb.dwarf2/imported-unit-bp.exp: Moved to ...
	* testsuite/gdb.dwarf2/imported-unit-bp.exp.tcl: ... here.
	* testsuite/gdb.dwarf2/imported-unit-bp-c++.exp: New test.
	* testsuite/gdb.dwarf2/imported-unit-bp-c.exp: New test.
	* testsuite/gdb.dwarf2/imported-unit.exp: Update.

gdb/ChangeLog

@@ -1,3 +1,13 @@
+2021-07-06  Tom de Vries  <tdevries@suse.de>
+
+	* dwarf2/read.c (scan_partial_symbols): Skip top-level imports of
+	c++ CU.
+	* testsuite/gdb.dwarf2/imported-unit-bp.exp: Moved to ...
+	* testsuite/gdb.dwarf2/imported-unit-bp.exp.tcl: ... here.
+	* testsuite/gdb.dwarf2/imported-unit-bp-c++.exp: New test.
+	* testsuite/gdb.dwarf2/imported-unit-bp-c.exp: New test.
+	* testsuite/gdb.dwarf2/imported-unit.exp: Update.
+
 2021-07-03  Joel Brobecker  <brobecker@adacore.com>
 
 	* NEWS: Create a new section for the next release branch.

gdb/dwarf2/read.c

@@ -7639,6 +7639,13 @@ scan_partial_symbols (struct partial_die_info *first_die, CORE_ADDR *lowpc,
 		  process_psymtab_comp_unit (per_cu, cu->per_objfile, true,
 					     cu->per_cu->lang);
 
+		if (pdi->die_parent == nullptr
+		    && per_cu->unit_type == DW_UT_compile
+		    && per_cu->lang == language_cplus)
+		  /* Regard import as hint.  See corresponding code in
+		     process_imported_unit_die.  */
+		  break;
+
 		cu->per_cu->imported_symtabs_push (per_cu);
 	      }
 	      break;

gdb/testsuite/gdb.dwarf2/imported-unit-bp-c++.exp

@@ -0,0 +1,18 @@
+# Copyright 2021 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+set lang DW_LANG_C_plus_plus
+
+source $srcdir/$subdir/imported-unit-bp.exp.tcl

gdb/testsuite/gdb.dwarf2/imported-unit-bp-c.exp

@@ -0,0 +1,18 @@
+# Copyright 2021 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+set lang DW_LANG_C
+
+source $srcdir/$subdir/imported-unit-bp.exp.tcl

gdb/testsuite/gdb.dwarf2/imported-unit-bp.exp.tcl

@@ -38,13 +38,14 @@ set asm_file [standard_output_file $srcfile2]
 Dwarf::assemble $asm_file {
     global srcdir subdir srcfile srcfile
     global build_options
+    global lang
     declare_labels lines_label callee_subprog_label cu_label
 
     get_func_info func "$build_options additional_flags=-DWITHMAIN"
 
     cu {} {
 	compile_unit {
-	    {language @DW_LANG_C}
+	    {language @$lang}
 	    {name "<artificial>"}
 	} {
 	    imported_unit {
@@ -56,7 +57,7 @@ Dwarf::assemble $asm_file {
     cu {} {
 	cu_label: compile_unit {
 	    {producer "gcc"}
-	    {language @DW_LANG_C}
+	    {language @$lang}
 	    {name ${srcfile}}
 	    {comp_dir "/tmp"}
 	    {low_pc 0 addr}

gdb/testsuite/gdb.dwarf2/imported-unit.exp

@@ -136,15 +136,14 @@ gdb_test_no_output "set language c++"
 
 set psymtabs_p [psymtabs_p]
 
-# Verify that the partial symtab for the unit importing the partial unit does
-# not contain the static partial symbol int, which is defined in the partial
-# unit.  Test-case for PR25646.
+# Verify that the partial symtab for CU "<artificial>" does
+# not contain the static partial symbol int, which is defined in the
+# CU "imported_unit.c".  Test-case for PR25646.
 set test "no static partial symbols in importing unit"
 if { $psymtabs_p } {
     gdb_test "main print psymbols" \
 	[multi_line \
-	     "  Depends on 1 other partial symtabs\." \
-	     "\[^\r\n\]*" \
+	 "  Depends on 0 other partial symtabs\." \
 	     "  Global partial symbols:" \
 	     "    `main', function, $hex" \
 	     "" \