[CIR] Missing `CK_BaseToDerived` emission in `emitCastLValue`

[tommymcm]

Minimized test case:

class B {};
class A : B {
  A &operator=(A &);
};
A &A::operator=(A &B) { (A &)B::operator=(B); }

Failed assertion:

clangir/clang/lib/CIR/CodeGen/CIRGenExpr.cpp

Line 2017 in aeac352

assert(0 && "NYI");

Flags: -fclangir -O2