[clang] [CIR] Upstream new SetBitfieldOp for handling C and C++ struct bitfields (PR #147609)

[Erich Keane via cfe-commits]

================
@@ -224,6 +224,10 @@ void CIRGenFunction::emitStoreThroughLValue(RValue src, LValue dst,
       return;
     }
 
+    assert(dst.isBitField() && "NIY LValue type");
----------------
erichkeane wrote:

```suggestion
    assert(dst.isBitField() && "NYI LValue type");
```

Also, why is this an assert here for an NYI?  This should use the errorNYI stuff instead, perhaps just put htis in a `isBitField` branch?

https://github.com/llvm/llvm-project/pull/147609