[CIR] Support CXXRewrittenBinaryOperator in emitLValue

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.

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:

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

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

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

Minimal Reproducer

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

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:

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():

case Expr::CXXRewrittenBinaryOperatorClass:
  return EmitLValue(cast<CXXRewrittenBinaryOperator>(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.