threading_support: refactor for Win32 threading

Refactor the header to allow us to implement alternate threading models
with alternate data structures.  Take the opportunity to clang-format
the area.  This will allow us to avoid re-declaring the interfaces for
Win32 threading.  NFC

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@290850 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed