Call __cxa_begin_catch before std::terminate() in a noexcept trap.

r174939-40 caused us to do this in the canonical terminate lpad,
but when the EH stack has other cleanups on it we use the
terminate handler block, which wasn't doing this.

Fixes the rest of rdar://11904428 given appropriate stdlib support.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@184475 91177308-0d34-0410-b5e6-96231b3b80d8