Don't remove C++ aliases from completions if symbol doesn't match

Don't remove C++ aliases from completions if symbol doesn't match

completion_list_add_symbol currently tries to remove C++ function
aliases from the completions match list even if the symbol passed down
wasn't successfully added to the completion list because it didn't
match.  I.e., we call cp_canonicalize_string_no_typedefs for each and
every C++ function in the program, which is useful work.  This patch
skips that useless work.

gdb/ChangeLog:
2020-05-24  Pedro Alves  <palves@redhat.com>

	* symtab.c (completion_list_add_name): Return boolean indication
	of whether the symbol matched.
	(completion_list_add_symbol): Don't try to remove C++ aliases if
	the symbol didn't match in the first place.
	* symtab.h (completion_list_add_name): Return bool.