[DeclCXX] Remove unknown external linkage specifications

Partial revert of r372681 "Support for DWARF-5 C++ language tags".

The change introduced new external linkage languages ("C++11" and
"C++14") which not supported in C++.

It also changed the definition of the existing enum to use the DWARF
constants. The problem is that "LinkageSpecDeclBits.Language" (the field
that reserves this enum) is actually defined as 3 bits length
(bitfield), which cannot contain the new DWARF constants. Defining the
enum as integer literals is more appropriate for maintaining valid
values.

Differential Revision: https://reviews.llvm.org/D69935

Author: ekatz

clang-tools-extra/modularize/Modularize.cpp

@@ -585,8 +585,6 @@ class CollectEntitiesVisitor
       LinkageLabel = "extern \"C\" {}";
       break;
     case LinkageSpecDecl::lang_cxx:
-    case LinkageSpecDecl::lang_cxx_11:
-    case LinkageSpecDecl::lang_cxx_14:
       LinkageLabel = "extern \"C++\" {}";
       break;
     }

clang/include/clang/AST/DeclCXX.h

@@ -42,7 +42,6 @@
 #include "llvm/ADT/PointerUnion.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/iterator_range.h"
-#include "llvm/BinaryFormat/Dwarf.h"
 #include "llvm/Support/Casting.h"
 #include "llvm/Support/Compiler.h"
 #include "llvm/Support/PointerLikeTypeTraits.h"
@@ -2758,15 +2757,8 @@ class LinkageSpecDecl : public Decl, public DeclContext {
   /// Represents the language in a linkage specification.
   ///
   /// The values are part of the serialization ABI for
-  /// ASTs and cannot be changed without altering that ABI.  To help
-  /// ensure a stable ABI for this, we choose the DW_LANG_ encodings
-  /// from the dwarf standard.
-  enum LanguageIDs {
-    lang_c = llvm::dwarf::DW_LANG_C,
-    lang_cxx = llvm::dwarf::DW_LANG_C_plus_plus,
-    lang_cxx_11 = llvm::dwarf::DW_LANG_C_plus_plus_11,
-    lang_cxx_14 = llvm::dwarf::DW_LANG_C_plus_plus_14
-  };
+  /// ASTs and cannot be changed without altering that ABI.
+  enum LanguageIDs { lang_c = 1, lang_cxx = 2 };
 
 private:
   /// The source location for the extern keyword.

clang/lib/AST/DeclPrinter.cpp

@@ -1011,19 +1011,12 @@ void DeclPrinter::VisitCXXRecordDecl(CXXRecordDecl *D) {
 
 void DeclPrinter::VisitLinkageSpecDecl(LinkageSpecDecl *D) {
   const char *l;
-  switch (D->getLanguage()) {
-  case LinkageSpecDecl::lang_c:
+  if (D->getLanguage() == LinkageSpecDecl::lang_c)
     l = "C";
-    break;
-  case LinkageSpecDecl::lang_cxx_14:
-    l = "C++14";
-    break;
-  case LinkageSpecDecl::lang_cxx_11:
-    l = "C++11";
-    break;
-  case LinkageSpecDecl::lang_cxx:
+  else {
+    assert(D->getLanguage() == LinkageSpecDecl::lang_cxx &&
+           "unknown language in linkage specification");
     l = "C++";
-    break;
   }
 
   Out << "extern \"" << l << "\" ";

clang/lib/AST/JSONNodeDumper.cpp

@@ -878,12 +878,6 @@ void JSONNodeDumper::VisitLinkageSpecDecl(const LinkageSpecDecl *LSD) {
   switch (LSD->getLanguage()) {
   case LinkageSpecDecl::lang_c: Lang = "C"; break;
   case LinkageSpecDecl::lang_cxx: Lang = "C++"; break;
-  case LinkageSpecDecl::lang_cxx_11:
-    Lang = "C++11";
-    break;
-  case LinkageSpecDecl::lang_cxx_14:
-    Lang = "C++14";
-    break;
   }
   JOS.attribute("language", Lang);
   attributeOnlyIfTrue("hasBraces", LSD->hasBraces());

clang/lib/AST/TextNodeDumper.cpp

@@ -1769,12 +1769,6 @@ void TextNodeDumper::VisitLinkageSpecDecl(const LinkageSpecDecl *D) {
   case LinkageSpecDecl::lang_cxx:
     OS << " C++";
     break;
-  case LinkageSpecDecl::lang_cxx_11:
-    OS << " C++11";
-    break;
-  case LinkageSpecDecl::lang_cxx_14:
-    OS << " C++14";
-    break;
   }
 }
 

clang/lib/CodeGen/CodeGenModule.cpp

@@ -5192,9 +5192,7 @@ void CodeGenModule::EmitObjCIvarInitializations(ObjCImplementationDecl *D) {
 // EmitLinkageSpec - Emit all declarations in a linkage spec.
 void CodeGenModule::EmitLinkageSpec(const LinkageSpecDecl *LSD) {
   if (LSD->getLanguage() != LinkageSpecDecl::lang_c &&
-      LSD->getLanguage() != LinkageSpecDecl::lang_cxx &&
-      LSD->getLanguage() != LinkageSpecDecl::lang_cxx_11 &&
-      LSD->getLanguage() != LinkageSpecDecl::lang_cxx_14) {
+      LSD->getLanguage() != LinkageSpecDecl::lang_cxx) {
     ErrorUnsupported(LSD, "linkage spec");
     return;
   }

clang/lib/Sema/SemaDeclCXX.cpp

@@ -14205,10 +14205,6 @@ Decl *Sema::ActOnStartLinkageSpecification(Scope *S, SourceLocation ExternLoc,
     Language = LinkageSpecDecl::lang_c;
   else if (Lang == "C++")
     Language = LinkageSpecDecl::lang_cxx;
-  else if (Lang == "C++11")
-    Language = LinkageSpecDecl::lang_cxx_11;
-  else if (Lang == "C++14")
-    Language = LinkageSpecDecl::lang_cxx_14;
   else {
     Diag(LangStr->getExprLoc(), diag::err_language_linkage_spec_unknown)
       << LangStr->getSourceRange();

clang/lib/Sema/SemaModule.cpp

@@ -31,8 +31,6 @@ static void checkModuleImportContext(Sema &S, Module *M,
         ExternCLoc = LSD->getBeginLoc();
       break;
     case LinkageSpecDecl::lang_cxx:
-    case LinkageSpecDecl::lang_cxx_11:
-    case LinkageSpecDecl::lang_cxx_14:
       break;
     }
     DC = LSD->getParent();