Change find_pcs_for_symtab_line to return a std::vector

Change find_pcs_for_symtab_line to return a std::vector

This changes find_pcs_for_symtab_line to return a std::vector.  This
allows the removal of some cleanups.

gdb/ChangeLog
2017-04-12  Tom Tromey  <tom@tromey.com>

	* symtab.h (find_pcs_for_symtab_line): Change return type.
	* symtab.c (find_pcs_for_symtab_line): Change return type.
	* python/py-linetable.c (build_line_table_tuple_from_pcs): Change
	type of "vec".  Update.
	(ltpy_get_pcs_for_line): Update.
	* linespec.c (decode_digits_ordinary): Update.