[PATCH] Remove support for enabling / disabling multi-threading at runtime.

[Chandler Carruth]

I'm still really not happy with the approach for the global lock. This is
completely different from the approach you and I discussed, on a separate
patch with a separate review thread. Please *do not* fork reviews in this
way. Please revert this and resume the code review on the original thread.
=/ I'm sorry if you're frustrated by the time it is taking to get this in,
but when one person has expressed specific concerns, I think it is a
mistake to move forward without sync-ing back up with them.

http://reviews.llvm.org/D4076