Enable optimization of sin / cos pair into call to __sincos_stret for iOS7+.

rdar://12856873
Patch by Evan Cheng, with a fix for rdar://13209539 by Tilmann Scheller

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@193942 91177308-0d34-0410-b5e6-96231b3b80d8