[TSan] Support Objective-C @synchronized with tagged pointers

[TSan] Support Objective-C @synchronized with tagged pointers

Summary:
Objective-C employs tagged pointers, that is, small objects/values may be encoded directly in the pointer bits. The resulting pointer is not backed by an allocation/does not point to a valid memory. TSan infrastructure requires a valid address for `Acquire/Release` and `Mutex{Lock/Unlock}`.
This patch establishes such a mapping via a "dummy allocation" for each encountered tagged pointer value.

Reviewers: dcoughlin, kubamracek, dvyukov, delcypher

Reviewed By: dvyukov

Subscribers: llvm-commits, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D56238

llvm-svn: 350556