[libc++] Include skipped headers in the --trace-includes output

By default, Clang does not include headers that are skipped due to
the include guard optimization in the --trace-includes output, which
breaks the use case that we were trying to use it for.

However, Clang does support the -fshow-skipped-includes flag, which
does exactly what we need and will result in an accurate include
graph.

As a fly-by fix, make sure that our includes don't differ between
-fexceptions and -fno-exceptions.

Differential Revision: https://reviews.llvm.org/D134829