c++: Don't allow type-constraint auto(x) [PR104752]

c++: Don't allow type-constraint auto(x) [PR104752]

104752 points out that

  template<class T>
  concept C = true;
  auto y = C auto(1);

is ill-formed as per [dcl.type.auto.deduct]: "For an explicit type conversion,
T is the specified type, which shall be auto." which doesn't allow
type-constraint auto.

	PR c++/104752

gcc/cp/ChangeLog:

	* semantics.cc (finish_compound_literal): Disallow auto{x} for
	is_constrained_auto.
	* typeck2.cc (build_functional_cast_1): Likewise.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp23/auto-fncast12.C: New test.