[DebugInfo] Fix the mismatching between C++ language tags and Dwarf versions.

Summary: The tags DW_LANG_C_plus_plus_14 and DW_LANG_C_plus_plus_11, introduced in Dwarf-5, are unexpected in previous versions. Fixing the mismathing doesn't have any drawbacks for any other debuggers, but helps dbx.

Reviewed By: aprantl, shchenz

Differential Revision: https://reviews.llvm.org/D99250

Author: Esme-Yi

clang/lib/CodeGen/CGDebugInfo.cpp

@@ -568,9 +568,9 @@ void CGDebugInfo::CreateCompileUnit() {
   if (LO.CPlusPlus) {
     if (LO.ObjC)
       LangTag = llvm::dwarf::DW_LANG_ObjC_plus_plus;
-    else if (LO.CPlusPlus14)
+    else if (LO.CPlusPlus14 && CGM.getCodeGenOpts().DwarfVersion >= 5)
       LangTag = llvm::dwarf::DW_LANG_C_plus_plus_14;
-    else if (LO.CPlusPlus11)
+    else if (LO.CPlusPlus11 && CGM.getCodeGenOpts().DwarfVersion >= 5)
       LangTag = llvm::dwarf::DW_LANG_C_plus_plus_11;
     else
       LangTag = llvm::dwarf::DW_LANG_C_plus_plus;

clang/test/CodeGenCXX/debug-info-programming-language.cpp

@@ -0,0 +1,13 @@
+// RUN: %clang_cc1 -dwarf-version=5  -emit-llvm -triple %itanium_abi_triple %s -o - \
+// RUN:   -x c++ -std=c++14 -O0 -disable-llvm-passes -debug-info-kind=limited \
+// RUN: | FileCheck --check-prefix=CHECK-DWARF5 %s
+// RUN: %clang_cc1 -dwarf-version=3  -emit-llvm -triple %itanium_abi_triple %s -o - \
+// RUN:   -x c++ -std=c++14 -O0 -disable-llvm-passes -debug-info-kind=limited \
+// RUN: | FileCheck --check-prefix=CHECK-DWARF3 %s
+
+int main() {
+  return 0;
+}
+
+// CHECK-DWARF5: distinct !DICompileUnit(language: DW_LANG_C_plus_plus_14,
+// CHECK-DWARF3: distinct !DICompileUnit(language: DW_LANG_C_plus_plus,

clang/test/Modules/ModuleDebugInfo.cpp

@@ -23,7 +23,7 @@
 // CHECK-MOD: distinct !DICompileUnit(language: DW_LANG_{{.*}}C_plus_plus,
 
 // CHECK: distinct !DICompileUnit(language: DW_LANG_{{.*}}C_plus_plus,
-// CHECK-CXX: distinct !DICompileUnit(language: DW_LANG_C_plus_plus_11,
+// CHECK-CXX: distinct !DICompileUnit(language: DW_LANG_C_plus_plus,
 // CHECK-SAME:                    isOptimized: false,
 // CHECK-NOT:                     splitDebugFilename:
 // CHECK-SAME:                    dwoId: