[gdb/symtab] Fix line-table end-of-sequence sorting

Consider test-case gdb.dwarf2/dw2-ranges-base.exp.  It has (ignoring
non-sensical entries that are filtered out by buildsym_compunit::record_line)
a line-table for dw2-ranges-base.c like this:
...
 Line Number Statements:
  [0x0000014e]  Extended opcode 2: set Address to 0x4004ba
  [0x00000159]  Advance Line by 10 to 11
  [0x0000015b]  Copy
  [0x0000015c]  Advance PC by 12 to 0x4004c6
  [0x0000015e]  Extended opcode 1: End of Sequence

  [0x00000161]  Extended opcode 2: set Address to 0x4004ae
  [0x0000016c]  Advance Line by 20 to 21
  [0x0000016e]  Copy
  [0x0000016f]  Advance PC by 12 to 0x4004ba
  [0x00000171]  Extended opcode 1: End of Sequence

  [0x00000174]  Extended opcode 2: set Address to 0x4004a7
  [0x0000017f]  Advance Line by 30 to 31
  [0x00000181]  Copy
  [0x00000182]  Advance PC by 7 to 0x4004ae
  [0x00000184]  Extended opcode 1: End of Sequence
...

If we disable the sorting in buildsym_compunit::end_symtab_with_blockvector,
we have the unsorted line table:
...
INDEX  LINE   ADDRESS            IS-STMT
0      11     0x00000000004004ba Y
1      END    0x00000000004004c6 Y
2      21     0x00000000004004ae Y
3      END    0x00000000004004ba Y
4      31     0x00000000004004a7 Y
5      END    0x00000000004004ae Y
...
It contains 3 sequences, 11/END, 21/END and 31/END.

We want to sort the 3 sequences relative to each other, while sorting on
address, to get:
...
INDEX  LINE   ADDRESS            IS-STMT
0      31     0x00000000004004a7 Y
1      END    0x00000000004004ae Y
2      21     0x00000000004004ae Y
3      END    0x00000000004004ba Y
4      11     0x00000000004004ba Y
5      END    0x00000000004004c6 Y
...

However, if we re-enable the sorting, we have instead:
...
INDEX  LINE   ADDRESS            IS-STMT
0      31     0x00000000004004a7 Y
1      21     0x00000000004004ae Y
2      END    0x00000000004004ae Y
3      11     0x00000000004004ba Y
4      END    0x00000000004004ba Y
5      END    0x00000000004004c6 Y
...

This is a regression since commit 3d92a3e313 "gdb: Don't reorder line table
entries too much when sorting", that introduced sorting on address while
keeping entries with the same address in pre-sort order.

Indeed the entries 1 and 2 are in pre-sort order (they map to entries 2 and 5
in the unsorted line table), but entry 1 does not belong in the sequence
terminated by 2.

Fix this by handling End-Of-Sequence entries in the sorting function, such
that they are sorted before other entries with the same address.

Also, revert the find_pc_sect_line workaround introduced in commit 3d92a3e313,
since that's no longer necessary.

Tested on x86_64-linux.

gdb/ChangeLog:

2020-07-06  Tom de Vries  <tdevries@suse.de>

	* buildsym.c (buildsym_compunit::end_symtab_with_blockvector): Handle
	End-Of-Sequence in lte_is_less_than.
	* symtab.c (find_pc_sect_line): Revert change from commit 3d92a3e313
	"gdb: Don't reorder line table entries too much when sorting".

gdb/testsuite/ChangeLog:

2020-07-06  Tom de Vries  <tdevries@suse.de>

	* gdb.dwarf2/dw2-ranges-base.exp: Test line-table order.

gdb/buildsym.c

@@ -943,6 +943,10 @@ buildsym_compunit::end_symtab_with_blockvector (struct block *static_block,
 	    = [] (const linetable_entry &ln1,
 		  const linetable_entry &ln2) -> bool
 	      {
+		if (ln1.pc == ln2.pc
+		    && ((ln1.line == 0) != (ln2.line == 0)))
+		  return ln1.line == 0;
+
 		return (ln1.pc < ln2.pc);
 	      };