CodeGens uses of @available into calls to the compiler-rt function
__isOSVersionAtLeast.

This commit is part of a feature that I proposed here:
http://lists.llvm.org/pipermail/cfe-dev/2016-July/049851.html

Differential revision: https://reviews.llvm.org/D27827

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@296015 91177308-0d34-0410-b5e6-96231b3b80d8