Remove strict tid checks from the mac implementation of BlockingMutex

Summary:
This patch unifies the behavior of BlockingMutex on linux and mac,
resolving problems that can arise when BlockingMutex is used in
code shared by the two platforms but has different behavior depending
on the platform.

No longer requires that the calling thread own the mutex for
CheckLocked calls to pass.

Reviewers: dvyukov, kubamracek

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D29728

llvm-svn: 294614