c++: constantness of call to function pointer [PR111703]

potential_constant_expression for CALL_EXPR tests FUNCTION_POINTER_TYPE_P
on the callee rather than on the type of the callee, which means we
always pass want_rval=any when recursing and so may fail to identify a
non-constant function pointer callee as such.  Fixing this turns out to
further work around PR111703.

	PR c++/111703
	PR c++/107939

gcc/cp/ChangeLog:

	* constexpr.cc (potential_constant_expression_1) <case CALL_EXPR>:
	Fix FUNCTION_POINTER_TYPE_P test.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-fn8.C: Extend test.
	* g++.dg/diagnostic/constexpr4.C: New test.

(cherry picked from commit 0077c0fb)