Fix a serious bug that would cause deadlock during abstract type refinement. The...

Fix a serious bug that would cause deadlock during abstract type refinement.  The constant creation
gets involved, and we end up trying to recursively acquire a writer lock.  The fix for this is slightly horrible,
and involves passing a boolean "locked" parameter around in Constants.cpp, but it's better than having locked and
unlocked versions of most of the code.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@73790 91177308-0d34-0410-b5e6-96231b3b80d8