Fix pr 17276.

Fix pr 17276.

See the description here:
https://sourceware.org/ml/gdb-patches/2014-08/msg00283.html

This patch keeps track of whether the current line has seen a
non-zero discriminator, and if so coalesces consecutive entries
for the same line (by ignoring all entries after the first).

gdb/ChangeLog:

	PR 17276
	* dwarf2read.c (dwarf_record_line_p): New function.
	(dwarf_decode_lines_1): Ignore subsequent line number entries
	for the same line if any entry had a non-zero discriminator.

gdb/testsuite/ChangeLog:

	* gdb.dwarf2/dw2-single-line-discriminators.S: New file.
	* gdb.dwarf2/dw2-single-line-discriminators.c: New file.
	* gdb.dwarf2/dw2-single-line-discriminators.exp: New file.