Handle libc++ usage of inline namespaces for std::

Handle libc++ usage of inline namespaces for std::

Summary:
- libc++ uses inline namespaces in its implementation of the std
namespace; use its macros for our forward-declaring needs.

@override-unit-failures
The hphp_packedtv test has been sitting postponed for a long time. I
can't repro its failure locally anyway.

Test Plan:
- fbconfig -r folly && fbmake runtests
- ./configure && make check on Ubuntu/FC/Mac

Reviewed By: simpkins@fb.com

FB internal diff: D998578