[llvm] [DirectX] Update CBuffer to refer to a `dx.Layout` type (PR #128697)

[Chris B via llvm-commits]

================
@@ -382,7 +382,15 @@ ResourceTypeInfo::UAVInfo ResourceTypeInfo::getUAV() const {
 
 uint32_t ResourceTypeInfo::getCBufferSize(const DataLayout &DL) const {
   assert(isCBuffer() && "Not a CBuffer");
-  return cast<CBufferExtType>(HandleTy)->getCBufferSize();
+
+  Type *ElTy = cast<CBufferExtType>(HandleTy)->getResourceType();
+
+  if (auto *LayoutTy = dyn_cast<LayoutExtType>(ElTy)) {
+    return LayoutTy->getSize();
+  }
----------------
llvm-beanz wrote:

nit:
```suggestion
  if (auto *LayoutTy = dyn_cast<LayoutExtType>(ElTy))
    return LayoutTy->getSize();
```

https://github.com/llvm/llvm-project/pull/128697