[ObjC][CodeGen] CodeGen support for @available.

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

llvm-svn: 296015