Thread Safety Analysis: add new warning flag, -Wthread-safety-reference, which
warns when a guarded variable is passed by reference as a function argument.
This is released as a separate warning flag, because it could potentially
break existing code that uses thread safety analysis.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@218087 91177308-0d34-0410-b5e6-96231b3b80d8