[CodeGenObjC] Emit [[X alloc] init] as objc_alloc_init(X) when available

This provides a code size win on the caller side, since the init
message send is done in the runtime function.

rdar://44987038

Differential revision: https://reviews.llvm.org/D57936

llvm-svn: 354056
diff --git a/clang/lib/CodeGen/CodeGenModule.h b/clang/lib/CodeGen/CodeGenModule.h
index 4139a0e..a652f14 100644
--- a/clang/lib/CodeGen/CodeGenModule.h
+++ b/clang/lib/CodeGen/CodeGenModule.h
@@ -124,6 +124,9 @@
   /// void objc_allocWithZone(id);
   llvm::FunctionCallee objc_allocWithZone;
 
+  /// void objc_alloc_init(id);
+  llvm::FunctionCallee objc_alloc_init;
+
   /// void objc_autoreleasePoolPop(void*);
   llvm::FunctionCallee objc_autoreleasePoolPop;