Reverts r289181: it's currently breaking modules using simd.h in
10.12 SDK.
This reverts commit 6e73e3464e96a4e00492c24aa790d36e1adb5702.
llvm-svn: 289487
Allow darwin to provide additional definitions and implementation
specifc values for tgmath.h on Apple platforms.
rdar://problem/19019845
llvm-svn: 289181