Remove some no-longer-needed workarounds in libc++abi

This CL:
* Removes libc++abi_warnings to workaround [1], since that bug was
  fixed
* Makes libc++abi always a source_set, and only excludes
  cxa_thread_atexit.cpp on mac (see [2] for context).
* Limits the visibility of libc++abi to libc++.  This is to prevent
  abusing including it like in [3].

[1] https://bugs.llvm.org/show_bug.cgi?id=25978
[2] https://bugs.llvm.org/show_bug.cgi?id=33272
[3] https://chromium.googlesource.com/chromium/deps/icu.git/+/2341038bf72869a5683a893a2b319a48ffec7f62

R=hans@chromium.org

Review-Url: https://codereview.chromium.org/2928503002 .
1 file changed