[Clang] Reintroduce obsolete libclang symbols to avoid an ABI break (#149079)

For more context, see
#119269 (comment),
but briefly, when removing ARCMigrate, I also removed some symbols in
libclang, which constitutes an ABI break that we don’t want, so this pr
reintroduces the removed symbols; the declarations are marked as
deprecated for future removal, and the implementations print an error
and do nothing, which is what we used to do when ARCMigrate was
disabled.

(cherry picked from commit 1600450)

Author: Sirraide

clang/include/clang-c/Index.h

@@ -6953,6 +6953,21 @@ clang_getCursorUnaryOperatorKind(CXCursor cursor);
  * @}
  */
 
+CINDEX_DEPRECATED
+typedef void *CXRemapping;
+
+CINDEX_DEPRECATED CINDEX_LINKAGE CXRemapping clang_getRemappings(const char *);
+
+CINDEX_DEPRECATED CINDEX_LINKAGE CXRemapping
+clang_getRemappingsFromFileList(const char **, unsigned);
+
+CINDEX_DEPRECATED CINDEX_LINKAGE unsigned clang_remap_getNumFiles(CXRemapping);
+
+CINDEX_DEPRECATED CINDEX_LINKAGE void
+clang_remap_getFilenames(CXRemapping, unsigned, CXString *, CXString *);
+
+CINDEX_DEPRECATED CINDEX_LINKAGE void clang_remap_dispose(CXRemapping);
+
 LLVM_CLANG_C_EXTERN_C_END
 
 #endif

clang/tools/libclang/CMakeLists.txt

@@ -42,6 +42,7 @@ set(SOURCES
   Indexing.cpp
   FatalErrorHandler.cpp
   Rewrite.cpp
+  Obsolete.cpp
 
   ADDITIONAL_HEADERS
   CIndexDiagnostic.h

clang/tools/libclang/Obsolete.cpp

@@ -0,0 +1,48 @@
+//===- Obsolete.cpp - Obsolete libclang functions and types -------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===--------------------------------------------------------------------===//
+//
+// This file contains libclang symbols whose underlying functionality has been
+// removed from Clang, but which need to be kept around so as to retain ABI
+// compatibility.
+//
+//===--------------------------------------------------------------------===//
+
+#include "clang-c/CXString.h"
+#include "clang-c/Index.h"
+#include "clang-c/Platform.h"
+#include "llvm/Support/raw_ostream.h"
+
+extern "C" {
+
+// The functions below used to be part of the C API for ARCMigrate, which has
+// since been removed from Clang; they already used to print an error if Clang
+// was compiled without arcmt support, so we continue doing so.
+CXRemapping clang_getRemappings(const char *) {
+  llvm::errs() << "error: ARCMigrate has been removed from Clang";
+  return nullptr;
+}
+
+CXRemapping clang_getRemappingsFromFileList(const char **, unsigned) {
+  llvm::errs() << "error: ARCMigrate has been removed from Clang";
+  return nullptr;
+}
+
+unsigned clang_remap_getNumFiles(CXRemapping) {
+  llvm::errs() << "error: ARCMigrate has been removed from Clang";
+  return 0;
+}
+
+void clang_remap_getFilenames(CXRemapping, unsigned, CXString *, CXString *) {
+  llvm::errs() << "error: ARCMigrate has been removed from Clang";
+}
+
+void clang_remap_dispose(CXRemapping) {
+  llvm::errs() << "error: ARCMigrate has been removed from Clang";
+}
+
+} // extern "C"

llvm/utils/gn/secondary/clang/tools/libclang/BUILD.gn

@@ -87,6 +87,7 @@ shared_library("libclang") {
     "Index_Internal.h",
     "Indexing.cpp",
     "Rewrite.cpp",
+    "Obsolete.cpp",
   ]
   if (host_os == "mac") {
     ldflags = [