c++: constinit on pointer to function [PR104066]

c++: constinit on pointer to function [PR104066]

[dcl.constinit]: "The constinit specifier shall be applied only to
a declaration of a variable with static or thread storage duration."

Thus, this ought to be OK:

  constinit void (*p)() = nullptr;

but the error message I introduced when implementing constinit was
not looking at funcdecl_p, so the code above was rejected.

Fixed thus.  I'm checking constinit_p first because I think that's
far more likely to be false than funcdecl_p.

	PR c++/104066

gcc/cp/ChangeLog:

	* decl.cc (grokdeclarator): Check funcdecl_p before complaining
	about constinit.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/constinit18.C: New test.

(cherry picked from commit 7b3b2f50953c5143d4b14b59d322d8a793f411dd)