Restore r184205 and associated commits (after commit of r185290)

This allows clang to use the backend parameter attribute 'returned' when generating 'this'-returning constructors and destructors in ARM and MSVC C++ ABIs.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@185291 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/CodeGen/CodeGenModule.cpp b/lib/CodeGen/CodeGenModule.cpp
index efff4b1..3298d5a 100644
--- a/lib/CodeGen/CodeGenModule.cpp
+++ b/lib/CodeGen/CodeGenModule.cpp
@@ -722,6 +722,14 @@
   if (!IsIncompleteFunction)
     SetLLVMFunctionAttributes(FD, getTypes().arrangeGlobalDeclaration(GD), F);
 
+  if (getCXXABI().HasThisReturn(GD)) {
+    assert(!F->arg_empty() &&
+           F->arg_begin()->getType()
+             ->canLosslesslyBitCastTo(F->getReturnType()) &&
+           "unexpected this return");
+    F->addAttribute(1, llvm::Attribute::Returned);
+  }
+
   // Only a few attributes are set on declarations; these may later be
   // overridden by a definition.