Don't adjust field offsets for external record layouts

[zturner]

Summary

injecting the VBPtr always behaves this way, but injecting the VFPtr was not. This patch fixes that.

[majnemer]

LGTM with a test added, see test/CodeGenCXX/override-layout.cpp for an example of how to test external record layout.

[majnemer]

It seems our infrastructure is not up to the task of making this testable via Clang, please commit.

lib/AST/RecordLayoutBuilder.cpp

   // alignment.
   CharUnits Offset = PointerInfo.Size.RoundUpToAlignment(
       std::max(RequiredAlignment, Alignment));
-  // Increase the size of the object and push back all fields, the vbptr and all
-  // bases by the offset amount.
+  // Push back the vbptr, but increase the size of the object and push back
+  // regular fields by the offset only if not using external record layout.
+  if (HasVBPtr)
+    VBPtrOffset += Offset;
+
+  if (UseExternalLayout)
+    return;
+
   Size += Offset;
+
+  // If we're using an external layout, the fields offsets have already
+  // accounted
+  // for this adjustment.
   for (uint64_t &FieldOffset : FieldOffsets)
     FieldOffset += Context.toBits(Offset);
-  if (HasVBPtr)
-    VBPtrOffset += Offset;
   for (BaseOffsetsMapTy::value_type &Base : Bases)
     Base.second += Offset;
 }