[libc++] Guard terminate_successful with TEST_HAS_NO_EXCEPTIONS

This one is a bit twisted. Some platforms don't have support for
exiting in a clean manner, so they don't provide std::exit(). As
a result, defining `terminate_successful()` on those platforms won't
work, and the PSTL tests that rely on `terminate_successful()` also
won't work.

However, we don't have a notion of "no clean termination" in libc++,
so we can't properly guard this. Since embedded platforms that don't
support clean termination usually also don't enable exceptions, we
don't need to be able to run those `terminate_successful` PSTL tests,
and guarding the definition of `terminate_successful` with
TEST_HAS_NO_EXCEPTIONS works pretty well.

This is kind of a hack for the lack of having a concept of "no clean
termination" in the library and in the test suite.

Differential Revision: https://reviews.llvm.org/D153302