Skip exception cleanups when the innermost scope is EHTerminateScope.

EHTerminateScope is used to implement C++ noexcept semantics. Per C++
[except.terminate], it is implemented-defined whether no, some, or all
cleanups are run prior to terminatation.

Therefore, the code to run cleanups on the way towards termination is
unnecessary, and may be omitted.

After this change, we will still run some cleanups: any cleanups in a
function called from the noexcept function will continue to run, while
those in the noexcept function itself will not.

(Commit attempt 2: check InnermostEHScope != stable_end() before accessing it.)

Differential Revision: https://reviews.llvm.org/D113620