Thread Safety Analysis:  Move -Wthread-safety-negative out of the
-Wthread-safety umbrella flag, pending updates to documentation.  The flag
works, but is likely to be confusing to existing users of -Wthread-safety.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@215679 91177308-0d34-0410-b5e6-96231b3b80d8