[CIR] Upstream handling for C++ default argument l-values (llvm#167999)

This adds handling emitting C++ default arguments as l-values.

Author: andykaylor

clang/lib/CIR/CodeGen/CIRGenFunction.cpp

@@ -928,6 +928,11 @@ LValue CIRGenFunction::emitLValue(const Expr *e) {
     assert(!cir::MissingFeatures::cleanupWithPreservedValues());
     return lv;
   }
+  case Expr::CXXDefaultArgExprClass: {
+    auto *dae = cast<CXXDefaultArgExpr>(e);
+    CXXDefaultArgExprScope scope(*this, dae);
+    return emitLValue(dae->getExpr());
+  }
   case Expr::ParenExprClass:
     return emitLValue(cast<ParenExpr>(e)->getSubExpr());
   case Expr::GenericSelectionExprClass:

clang/test/CIR/CodeGen/defaultarg.cpp

@@ -0,0 +1,32 @@
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -fclangir -emit-cir -std=c++17 %s -o %t.cir
+// RUN: FileCheck %s --input-file=%t.cir --check-prefix=CIR
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -fclangir -emit-llvm -std=c++17 %s -o %t-cir.ll
+// RUN: FileCheck %s --input-file=%t-cir.ll --check-prefix=LLVM
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -emit-llvm -std=c++17 %s -o %t.ll
+// RUN: FileCheck %s --input-file=%t.ll --check-prefix=OGCG
+
+void bar(const int &i = 42);
+
+void foo() {
+  bar();
+}
+
+// CIR: cir.func {{.*}} @_Z3foov()
+// CIR:   cir.scope {
+// CIR:     %[[TMP0:.*]] = cir.alloca !s32i, !cir.ptr<!s32i>, ["ref.tmp0"]
+// CIR:     %[[TMP1:.*]] = cir.const #cir.int<42>
+// CIR:     cir.store{{.*}} %[[TMP1]], %[[TMP0]]
+// CIR:     cir.call @_Z3barRKi(%[[TMP0]])
+// CIR:   }
+
+// LLVM: define{{.*}} @_Z3foov()
+// LLVM:   %[[TMP0:.*]] = alloca i32
+// LLVM:   br label %[[SCOPE_LABEL:.*]]
+// LLVM: [[SCOPE_LABEL]]:
+// LLVM:   store i32 42, ptr %[[TMP0]]
+// LLVM:   call void @_Z3barRKi(ptr %[[TMP0]])
+
+// OGCG: define{{.*}} @_Z3foov()
+// OGCG:   %[[TMP0:.*]] = alloca i32
+// OGCG:   store i32 42, ptr %[[TMP0]]
+// OGCG:   call void @_Z3barRKi(ptr {{.*}} %[[TMP0]])