Thread safety analysis: Add a new "beta" warning flag: -Wthread-safety-beta.

As the analysis improves, it will continue to add new warnings that are
potentially disruptive to existing users.  From now on, such warnings will
first be introduced under the "beta" flag.  Such warnings are not turned on by
default; their purpose is to allow users to test their code against future
planned changes, before those changes are actually made.  After a suitable
migration period, beta warnings will be folded into the standard
-Wthread-safety.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@169338 91177308-0d34-0410-b5e6-96231b3b80d8