Recommit r339943 - Establish the <bit> header. NFC yet. Reviewed as https://reviews.llvm.org/D50815 - with a fix for the sanitizer bots

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@340045 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed