c++: module directive FSM

c++: module directive FSM

As mentioned in the preprocessor patches, there's a new kind of
preprocessor directive for modules, and it interacts with the
compiler-proper, as that has to stream in header-unit macro
information (when the directive is an import that names a
header-unit).  This is that machinery.  It's an FSM that inspects the
token stream and does the minimal parsing to detect such imports.
This ends up being called from the C++ parser's tokenizer and from the
-E tokenizer (via a lang hook).  The actual module streaming is a stub
here.

	gcc/cp/
	* cp-tree.h (module_token_pre, module_token_cdtor)
	(module_token_lang): Declare.
	* lex.c: Include langhooks.
	(struct module_token_filter): New.
	* cp-tree.h (module_token_pre, module_token_cdtor)
	(module_token_lang): Define.
	* module.cc (get_module, preprocess_module, preprocessed_module):
	Nop stubs.