preprocessor: Add deferred macros

Deferred macros are needed for C++ modules.  Header units may export
macro definitions and undefinitions.  These are resolved lazily at the
point of (potential) use.  (The language specifies that, it's not just
a useful optimization.)  Thus, identifier nodes grow a 'deferred'
field, which fortunately doesn't expand the structure on 64-bit
systems as there was padding there.  This is non-zero on NT_MACRO
nodes, if the macro is deferred.  When such an identifier is lexed, it
is resolved via a callback that I added recently.  That will either
provide the macro definition, or discover it there was an overriding
undef.  Either way the identifier is no longer a deferred macro.
Notice it is now possible for NT_MACRO nodes to have a NULL macro
expansion.

	libcpp/
	* include/cpplib.h (struct cpp_hashnode): Add deferred field.
	(cpp_set_deferred_macro): Define.
	(cpp_get_deferred_macro): Declare.
	(cpp_macro_definition): Reformat, add overload.
	(cpp_macro_definition_location): Deal with deferred macro.
	(cpp_alloc_token_string, cpp_compare_macro): Declare.
	* internal.h (_cpp_notify_macro_use): Return bool
	(_cpp_maybe_notify_macro_use): Likewise.
	* directives.c (do_undef): Check macro is not undef before
	warning.
	(do_ifdef, do_ifndef): Deal with deferred macro.
	* expr.c (parse_defined): Likewise.
	* lex.c (cpp_allocate_token_string): Break out of ...
	(create_literal): ... here.  Call it.
	(cpp_maybe_module_directive): Deal with deferred macro.
	* macro.c (cpp_get_token_1): Deal with deferred macro.
	(warn_of_redefinition): Deal with deferred macro.
	(compare_macros): Rename to ...
	(cpp_compare_macro): ... here.  Make extern.
	(cpp_get_deferred_macro): New.
	(_cpp_notify_macro_use): Deal with deferred macro, return bool
	indicating definedness.
	(cpp_macro_definition): Deal with deferred macro.

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