c++: don't wrap cleanups that can't throw

c++: don't wrap cleanups that can't throw

Since C++11, the vast majority of destructors are noexcept, so
wrap_temporary_cleanups adds a bunch of useless TRY_CATCH_EXPR to be removed
later in the optimizers.  It's simple to avoid adding them in the first
place.

gcc/cp/ChangeLog:

	* decl.c (wrap_cleanups_r): Don't wrap if noexcept.

gcc/testsuite/ChangeLog:

	* g++.dg/eh/cleanup6.C: New test.