Provide diagnostic hints for missing C inttypes.h string constants.

Provide diagnostic hints for missing C inttypes.h string constants.

This adds a flag to c_parser so we know when we were trying to
construct a string literal. If there is a parse error and we were
constructing a string literal, and the next token is an unknown
identifier name, and we know there is a standard header that defines
that name as a string literal, then add a missing header hint to
the error messsage.

The list of macro names are also used when providing a hint for
missing identifiers.

gcc/c-family/ChangeLog:

	* known-headers.cc (get_string_macro_hint): New function.
	(get_stdlib_header_for_name): Use get_string_macro_hint.
	(get_c_stdlib_header_for_string_macro_name): New function.
	* known-headers.h (get_c_stdlib_header_for_string_macro_name):
	New function declaration.

gcc/c/ChangeLog:

	* c-parser.c (struct c_parser): Add seen_string_literal
	bitfield.
	(c_parser_consume_token): Reset seen_string_literal.
	(c_parser_error_richloc): Add name_hint if seen_string_literal
	and next token is a CPP_NAME and we have a missing header
	suggestion for the name.
	(c_parser_string_literal): Set seen_string_literal.

gcc/testsuite/ChangeLog:

	* gcc.dg/spellcheck-inttypes.c: New test.
	* g++.dg/spellcheck-inttypes.C: Likewise.