[clang] [CIR] Fix access to bitfields inside a union (PR #154398)

[via cfe-commits]

================
@@ -372,9 +372,11 @@ Address CIRGenFunction::getAddrOfBitFieldStorage(LValue base,
                                                  unsigned index) {
   mlir::Location loc = getLoc(field->getLocation());
   cir::PointerType fieldPtr = cir::PointerType::get(fieldType);
+  auto rec = cast<cir::RecordType>(base.getAddress().getElementType());
+  if (index == 0 && rec.isUnion())
----------------
Andres-Salamanca wrote:

```diff
-      loc, fieldPtr, base.getPointer(), field->getName(), index);
+      loc, fieldPtr, base.getPointer(), field->getName(), rec.isUnion() ? field->getFieldIndex() : index);
```
I changed it to use `field->getFieldIndex()`, similar to how `CIRGenFunction::emitAddrOfFieldStorage` does.

https://github.com/llvm/llvm-project/pull/154398