c++: error with constexpr operator() [PR107939]

Similarly to PR107938, this also started with r11-557, whereby cp_finish_decl
can call check_initializer even in a template for a constexpr initializer.

Here we are rejecting

  extern const Q q;

  template<int>
  constexpr auto p = q(0);

even though q has a constexpr operator().  It's deemed non-const by
decl_maybe_constant_var_p because even though 'q' is const it is not
of integral/enum type.

If fun is not a function pointer, we don't know if we're using it as an
lvalue or rvalue, so with this patch we pass 'any' for want_rval.  With
that, p_c_e/VAR_DECL doesn't flat out reject the underlying VAR_DECL.

	PR c++/107939

gcc/cp/ChangeLog:

	* constexpr.cc (potential_constant_expression_1) <case CALL_EXPR>: Pass
	'any' when recursing on a VAR_DECL and not a pointer to function.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/var-templ74.C: Remove dg-error.
	* g++.dg/cpp1y/var-templ77.C: New test.