c++: Fix noexcept with unevaluated operand [PR101087]

c++: Fix noexcept with unevaluated operand [PR101087]

It sounds plausible that this assert

  int f();
  static_assert(noexcept(sizeof(f())));

should pass: sizeof produces a std::size_t and its operand is not
evaluated, so it can't throw.  noexcept should only evaluate to
false for potentially evaluated operands.  Therefore I think that
check_noexcept_r shouldn't walk into operands of sizeof/decltype/
alignof/typeof.

	PR c++/101087

gcc/cp/ChangeLog:

	* cp-tree.h (unevaluated_p): New.
	* except.c (check_noexcept_r): Use it.  Don't walk into
	unevaluated operands.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/noexcept70.C: New test.