C++20 modules introduces a new kind of preprocessor directive -- a
module directive.  These are directives but without the leading '#'.
We have to detect them by sniffing the start of a logical line.  When
detected we replace the initial identifiers with unspellable tokens
and pass them through to the language parser the same way deferred
pragmas are.  There's a PRAGMA_EOL at the logical end of line too.

One additional complication is that we have to do header-name lexing
after the initial tokens, and that requires changes in the macro-aware
piece of the preprocessor.  The above sniffer sets a counter in the
lexer state, and that triggers at the appropriate point.  We then do
the same header-name lexing that occurs on a #include directive or
has_include pseudo-macro.  Except that the header name ends up in the
token stream.

A couple of token emitters need to deal with the new token possibility.

	gcc/c-family/
	* c-lex.c (c_lex_with_flags): CPP_HEADER_NAMEs can now be seen.
	libcpp/
	* include/cpplib.h (struct cpp_options): Add module_directives
	option.
	(NODE_MODULE): New node flag.
	(struct cpp_hashnode): Make rid-code a bitfield, increase bits in
	flags and swap with type field.
	* init.c (post_options): Create module-directive identifier nodes.
	* internal.h (struct lexer_state): Add directive_file_token &
	n_modules fields.  Add module node enumerator.
	* lex.c (cpp_maybe_module_directive): New.
	(_cpp_lex_token): Call it.
	(cpp_output_token): Add '"' around CPP_HEADER_NAME token.
	(do_peek_ident, do_peek_module): New.
	(cpp_directives_only): Detect module-directive lines.
	* macro.c (cpp_get_token_1): Deal with directive_file_token
	triggering.