[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
diff --git a/clang/lib/CodeGen/CodeGenFunction.h b/clang/lib/CodeGen/CodeGenFunction.h
index 252561f..3bb8088 100644
--- a/clang/lib/CodeGen/CodeGenFunction.h
+++ b/clang/lib/CodeGen/CodeGenFunction.h
@@ -3169,6 +3169,8 @@
 public:
   llvm::Value *EmitMSVCBuiltinExpr(MSVCIntrin BuiltinID, const CallExpr *E);
 
+  llvm::Value *EmitBuiltinAvailable(ArrayRef<llvm::Value *> Args);
+
   llvm::Value *EmitObjCProtocolExpr(const ObjCProtocolExpr *E);
   llvm::Value *EmitObjCStringLiteral(const ObjCStringLiteral *E);
   llvm::Value *EmitObjCBoxedExpr(const ObjCBoxedExpr *E);