[gdb/testsuite] Add untested case in gdb.gdb/complaints.exp

When building gdb with "-Wall -O2 -g -flto=auto", I run into:
...
(gdb) call clear_complaints()^M
No symbol "clear_complaints" in current context.^M
(gdb) FAIL: gdb.gdb/complaints.exp: clear complaints
...

The problem is that lto has optimized away clear_complaints, and consequently
the selftests cannot run.

Fix this by:
- using info function to detect presence of clear_complaints
- handling the absence of clear_complaints by calling untested
...
(gdb) UNTESTED: gdb.gdb/complaints.exp: \
  Cannot find clear_complaints, skipping test
...

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2021-09-03  Tom de Vries  <tdevries@suse.de>

	* gdb.gdb/complaints.exp: Use untested if clear_complaints cannot
	be found.

gdb/testsuite/gdb.gdb/complaints.exp

@@ -104,6 +104,26 @@ proc test_empty_complaint { cmd msg } {
 }
 
 proc test_empty_complaints { } {
+    global decimal
+
+    set re [multi_line \
+		"All functions matching regular expression \[^:\]*:" \
+		"" \
+		"File \[^\r\n\]*/complaints\\.c:" \
+		"$decimal:\tvoid clear_complaints\\(\\);"]
+
+    set found 0
+    gdb_test_multiple "info function ^clear_complaints()$" "" {
+	-re -wrap $re {
+	    set found 1
+	}
+	-re -wrap "" {
+	}
+    }
+    if { ! $found } {
+	untested "Cannot find clear_complaints, skipping test"
+	return 0
+    }
 
     test_empty_complaint "call clear_complaints()" \
 	    "clear complaints"