Handle lack of <bits/c++config.h> and <bits/functexcept.h>

Handle lack of <bits/c++config.h> and <bits/functexcept.h>

Summary:
- Clang's libc++ doesn't provide these header files. Detect libc++ via
the _LIBCPP_VERSION symbol (pulling it in by sourcing some header files
earlier if necessary) and avoid using these files.
- This is another attempt at D1074481.

Test Plan: .

Reviewed By: andrei.alexandrescu@fb.com

FB internal diff: D1074723