c++: ICE with noexcept and local specialization [PR114114]

c++: ICE with noexcept and local specialization [PR114114]

Here we ICE because we call register_local_specialization while
local_specializations is null, so

  local_specializations->put ();

crashes on null this.  It's null since maybe_instantiate_noexcept calls
push_to_top_level which creates a new scope.  Normally, I would have
guessed that we need a new local_specialization_stack.  But here we're
dealing with an operand of a noexcept, which is an unevaluated operand,
and those aren't registered in the hash map.  maybe_instantiate_noexcept
wasn't signalling that it's substituting an unevaluated operand though.

	PR c++/114114

gcc/cp/ChangeLog:

	* pt.cc (maybe_instantiate_noexcept): Save/restore
	cp_unevaluated_operand, c_inhibit_evaluation_warnings, and
	cp_noexcept_operand around the tsubst_expr call.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/noexcept84.C: New test.