coroutines : Adjust error handling for type-dependent coroutines [PR96251].

coroutines : Adjust error handling for type-dependent coroutines [PR96251].

Although coroutines are not permitted to be constexpr, generic lambdas
are implicitly from C++17 and, because of this, a generic coroutine lambda
can be marked as potentially constexpr. As per the PR, this then fails when
type substitution is attempted because the check disallowing constexpr in
the coroutines code was overly restrictive.

This changes the error handing to mark the function  as 'invalid_constexpr'
but suppresses the error in the case that we are instantiating a constexpr.

gcc/cp/ChangeLog:

	PR c++/96251
	* coroutines.cc (coro_common_keyword_context_valid_p): Suppress
	error reporting when instantiating for a constexpr.

gcc/testsuite/ChangeLog:

	PR c++/96251
	* g++.dg/coroutines/pr96251.C: New test.