c++: ICE with throw inside concept [PR112437]

c++: ICE with throw inside concept [PR112437]

We crash in the loop at the end of treat_lvalue_as_rvalue_p for code
like

  template <class T>
  concept Throwable = requires(T x) { throw x; };

because the code assumes that we eventually reach sk_function_parms or
sk_try and bail, but in a concept we're in a sk_namespace.

We're already checking sk_try so we don't crash in a function-try-block,
but I've added a test anyway.

	PR c++/112437

gcc/cp/ChangeLog:

	* typeck.cc (treat_lvalue_as_rvalue_p): Bail out on sk_namespace in
	the move on throw of parms loop.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-throw1.C: New test.
	* g++.dg/eh/throw4.C: New test.

(cherry picked from commit 65b105b4f399559685200e1598ead8c7d0935c04)