[AST] Introduce a new DecayedType sugar node

The goal of this sugar node is to be able to look at an arbitrary
FunctionType and tell if any of the parameters were decayed from an
array or function type.  Ultimately this is necessary to implement
Microsoft's C++ name mangling scheme, which mangles decayed arrays
differently from normal pointers.

Reviewers: rsmith

Differential Revision: http://llvm-reviews.chandlerc.com/D1014

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@184763 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Sema/TreeTransform.h b/lib/Sema/TreeTransform.h
index 86624c5..9fbbe2c 100644
--- a/lib/Sema/TreeTransform.h
+++ b/lib/Sema/TreeTransform.h
@@ -3586,6 +3586,22 @@
 }
 
 template<typename Derived>
+QualType TreeTransform<Derived>::TransformDecayedType(TypeLocBuilder &TLB,
+                                                      DecayedTypeLoc TL) {
+  QualType OriginalType = getDerived().TransformType(TLB, TL.getOriginalLoc());
+  if (OriginalType.isNull())
+    return QualType();
+
+  QualType Result = TL.getType();
+  if (getDerived().AlwaysRebuild() ||
+      OriginalType != TL.getOriginalLoc().getType())
+    Result = SemaRef.Context.getDecayedType(OriginalType);
+  TLB.push<DecayedTypeLoc>(Result);
+  // Nothing to set for DecayedTypeLoc.
+  return Result;
+}
+
+template<typename Derived>
 QualType TreeTransform<Derived>::TransformPointerType(TypeLocBuilder &TLB,
                                                       PointerTypeLoc TL) {
   QualType PointeeType