Get rid of VEC(filename_language)

Get rid of VEC(filename_language)

This patch removes VEC(filename_language), replacing its usage with
std::vector.  filename_language::ext is changed to an std::string at the
same time.

Regtested on the buildbot.

gdb/ChangeLog:

	* symfile.c (filename_language): Make struct, not typedef.  Add
	constructor.
	<ext>: Change type to std::string.
	(DEF_VEC_O (filename_language)): Remove.
	(filename_language_table): Change type to std::vector.
	(add_filename_language): Adjust.
	(set_ext_lang_command): Adjust.
	(info_ext_lang_command): Adjust.
	(deduce_language_from_filename): Adjust.
	(class scoped_restore_filename_language_table): Remove.
	(test_filename_language): Use scoped_restore.
	(test_set_ext_lang_command): Use scoped_restore, adjust to
	std::vector change.