Add support for libc++

Currently cpp-std-fwd does not support LLVM's libc++.
Specifically,
`clang -stdlib=libc++ ...` does not work.
This is mostly because libc++ uses dedicated macros to mark the start and the end of the `std` namespace:
```
_LIBCPP_BEGIN_NAMESPACE_STD
_LIBCPP_END_NAMESPACE_STD
```