c++: Minor cleanup in parser.cc

c++: Minor cleanup in parser.cc

The code to determine whether a given token starts a module directive is
currently repeated in 4 places in parser.cc. I am about to submit a patch
that needs to add it in a 5th place, so since the code is not completely
trivial (needing to check for 3 different token types), it seems worthwhile
to factor this logic into its own function.

gcc/cp/ChangeLog:

	* parser.cc (cp_token_is_module_directive): New function
	refactoring common code.
	(cp_parser_skip_to_closing_parenthesis_1): Use the new function.
	(cp_parser_skip_to_end_of_statement): Likewise.
	(cp_parser_skip_to_end_of_block_or_statement): Likewise.
	(cp_parser_declaration): Likewise.