[CIR][CIRGen] Fix CXX codegen for default args (#1278)

This PR adds a support for for default arguments

Author: gitoleg

clang/lib/CIR/CodeGen/CIRGenExpr.cpp

@@ -2636,6 +2636,11 @@ LValue CIRGenFunction::emitLValue(const Expr *E) {
     // bitfield lvalue or some other non-simple lvalue?
     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::DeclRefExprClass:

clang/test/CIR/CodeGen/defaultarg.cpp

@@ -0,0 +1,12 @@
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -fclangir -emit-cir -std=c++17 %s -o - | FileCheck %s
+
+void bar(const int &i = 42);
+
+void foo() {
+  bar();
+}
+
+// CHECK: [[TMP0:%.*]] = cir.alloca !s32i
+// CHECK: [[TMP1:%.*]] = cir.const #cir.int<42>
+// CHECK: cir.store [[TMP1]], [[TMP0]]
+// CHECK: cir.call @_Z3barRKi([[TMP0]])