c++: TINFO_VAR_DECLARED_CONSTINIT -> DECL_DECLARED_CONSTINIT_P

c++: TINFO_VAR_DECLARED_CONSTINIT -> DECL_DECLARED_CONSTINIT_P

We need to record whether template function-scopestatic decls are
constinit.  That's currently held on the var's TEMPLATE_INFO data.
But I want to get rid of such decl's template header as they're not
really templates, and they're never instantiated separately from their
containing function's definition.  (Just like auto vars, which don't
get them for instance).

This patch moves the flag into a spare decl_lang_flag.

	gcc/cp/
	* cp-tree.h (TINFO_VAR_DECLARED_CONSTINIT): Replace with ...
	(DECL_DECLARED_CONSTINIT_P): ... this.
	* decl.c (start_decl): No need to retrofit_lang_decl for constinit
	flag.
	(cp_finish_decl): Use DECL_DECLARED_CONSTINIT_P.
	* pt.c (tsubst_decl): No need to handle constinit flag
	propagation.
	(tsubst_expr): Or here.