[tsan] Fix crash in objc_sync_enter/objc_sync_exit when using an Obj-C tagged pointer

Objective-C tagged pointers (either bottom-most or top-most bit is 1) are valid Obj-C objects but are not valid pointers. Make sure we don't crash on them when used in objc_sync_enter/objc_sync_exit. Instead, let's synchronize on a global object.

Differential Revision: https://reviews.llvm.org/D49707

llvm-svn: 337837