[Clang][NFC] assert IFaceT in SemaObjC::HandleExprPropertyRefExpr (#138026)
Static analysis flagged that we use IFaceT in HandleExprPropertyRefExpr
without checking even though getInterfaceType() can return nullptr. The
comments make it clear the assumption is that we will always have an
interface, so we will document this via an assert.
Fixes: https://github.com/llvm/llvm-project/issues/134954