Removing a bit of custom parsing functionality used by the thread safety analysis...

Removing a bit of custom parsing functionality used by the thread safety analysis APIs. Now using tablegen to determine whether an attribute's arguments should be parsed in an unevaluated context instead of relying on a separate, hard-coded list of attributes.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@198883 91177308-0d34-0410-b5e6-96231b3b80d8