[asan] Support dead code stripping on Mach-O platforms

On OS X El Capitan and iOS 9, the linker supports a new section
attribute, live_support, which allows dead stripping to remove dead
globals along with the ASAN metadata about them.

With this change __asan_global structures are emitted in a new
__DATA,__asan_globals section on Darwin.

Additionally, there is a __DATA,__asan_liveness section with the
live_support attribute. Each entry in this section is simply a tuple
that binds together the liveness of a global variable and its ASAN
metadata structure. Thus the metadata structure will be alive if and
only if the global it references is also alive.

Review: http://reviews.llvm.org/D16737
llvm-svn: 264645