[CIR] Support CXXRewrittenBinaryOperator in emitLValue

### Description

When generating ClangIR for a C++20 rewritten binary operator (e.g. rewritten comparison using `operator<=>`) whose evaluation produces an lvalue (such as binding a reference to the result), ClangIR emits a "Not Yet Implemented" error:

```text
error: ClangIR code gen Not Yet Implemented: emitLValue: CXXRewrittenBinaryOperator
```

This is located in `clang/lib/CIR/CodeGen/CIRGenFunction.cpp` under `CIRGenFunction::emitLValue`:

```cpp
case Expr::CXXRewrittenBinaryOperatorClass:
getCIRGenModule().errorNYI(e->getSourceRange(),
"emitLValue: CXXRewrittenBinaryOperator");
return LValue();
```

### Minimal Reproducer

Compiling the following C++20 test case with `-fclangir -emit-cir`:

```cpp
struct SpaceshipLValueResult {
int &operator<(int) const;
};

struct LValueItem {
SpaceshipLValueResult operator<=>(const LValueItem &) const;
};

void cxx_rewritten_binary_operator_lvalue_expr() {
LValueItem a;
LValueItem b;
int &ref = (a < b);
}
```

Output:
```text
error: ClangIR code gen Not Yet Implemented: emitLValue: CXXRewrittenBinaryOperator
```

### Reference Implementation

In classic Clang CodeGen (`clang/lib/CodeGen/CGExpr.cpp`), `CXXRewrittenBinaryOperator` evaluates its underlying semantic form via `getSemanticForm()`:

```cpp
case Expr::CXXRewrittenBinaryOperatorClass:
return EmitLValue(cast(E)->getSemanticForm(),
IsKnownNonNull);
```

Other expression emitters in ClangIR already implement `CXXRewrittenBinaryOperator` by delegating to `getSemanticForm()`:
- `clang/lib/CIR/CodeGen/CIRGenExprScalar.cpp` (`VisitCXXRewrittenBinaryOperator`)
- `clang/lib/CIR/CodeGen/CIRGenExprAggregate.cpp` (`VisitCXXRewrittenBinaryOperator`)
- `clang/lib/CIR/CodeGen/CIRGenExprComplex.cpp` (`VisitCXXRewrittenBinaryOperator`)

Supporting `CXXRewrittenBinaryOperator` in `CIRGenFunction::emitLValue` (`clang/lib/CIR/CodeGen/CIRGenFunction.cpp`) and adding corresponding test cases to `clang/test/CIR/CodeGen/cxx-rewritten-binary-operator.cpp` will bring parity with classic CodeGen and complete coverage for rewritten binary operators across all CIR expression emitters.

Start in clang/lib/CIR/CodeGen/CIRGenFunction.cpp at CIRGenFunction::emitLValue and compare the classic Clang handling in clang/lib/CodeGen/CGExpr.cpp. Review the existing rewritten-operator visitors in CIRGenExprScalar.cpp, CIRGenExprAggregate.cpp, and CIRGenExprComplex.cpp, then add coverage in clang/test/CIR/CodeGen/cxx-rewritten-binary-operator.cpp. Done means the reproducer no longer emits the Not Yet Implemented error and the test passes.

Written by the indexing model from the issue text.