[asan] Turn on Mach-O global metadata liveness tracking by default

This patch turns on the metadata liveness tracking since all known issues
have been resolved. The future has been implemented in
https://reviews.llvm.org/D16737 and enables support of dead code stripping
option on Mach-O platforms.

As part of enabling the feature, I also plan on reverting the following
patch to compiler-rt:

http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20160704/369910.html

Differential Revision: https://reviews.llvm.org/D26772

llvm-svn: 287235