Provide diagnostic hints for missing C++ cinttypes string constants.

Provide diagnostic hints for missing C++ cinttypes string constants.

When reporting an error in cp_parser and we notice a string literal
followed by an unknown name check whether there is a known standard
header containing a string macro with the same name, then add a hint
to the error message to include that header.

gcc/c-family/ChangeLog:

	* known-headers.cc (get_cp_stdlib_header_for_string_macro_name):
	New function.
	* known-headers.h (get_cp_stdlib_header_for_string_macro_name):
	New function declaration.

gcc/cp/ChangeLog:

	* parser.c (cp_lexer_safe_previous_token): New function.
	(cp_parser_error_1): Add name_hint if the previous token is
	a string literal and next token is a CPP_NAME and we have a
	missing header suggestion for the name.

gcc/testsuite/ChangeLog:

	* g++.dg/spellcheck-inttypes.C: Add string-literal testcases.