Add a flag that controls if clang-tidy and clang-include-fixer are built into libclang.

Based on the discussion on D55415, also make the flag default to false.
Having libclang depend on clang-tools-extra means check-clang builds all
of clang-tools-extra, which besides being a layering violation takes
quite some time, since clang-tools-extra has many files that are slow
to compile.

Longer term, we likely will want to remove this flag completely. If
people need this functionality, maybe there could be a
libclang-tools-extra that's libclang + clang-tidy and
clang-includes-fixer linked in.

Differential Revision: https://reviews.llvm.org/D79599

clang/docs/ReleaseNotes.rst

@@ -240,7 +240,11 @@ Build System Changes
 These are major changes to the build system that have happened since the 10.0.0
 release of Clang. Users of the build system should adjust accordingly.
 
-- ...
+- clang-tidy and clang-include-fixer are no longer compiled into libclang by
+  default. You can set ``LIBCLANG_INCLUDE_CLANG_TOOLS_EXTRA=ON`` to undo that,
+  but it's expected that that setting will go away eventually. If this is
+  something you need, please reach out to the mailing list to discuss possible
+  ways forward.
 
 AST Matchers
 ------------

clang/tools/libclang/CMakeLists.txt

@@ -50,13 +50,20 @@ if (CLANG_ENABLE_ARCMT)
   list(APPEND LIBS clangARCMigrate)
 endif ()
 
-if (TARGET clangTidyPlugin)
-  add_definitions(-DCLANG_TOOL_EXTRA_BUILD)
-  list(APPEND LIBS clangTidyPlugin)
-  list(APPEND LIBS clangIncludeFixerPlugin)
-  if(LLVM_ENABLE_MODULES)
-    list(APPEND LLVM_COMPILE_FLAGS "-fmodules-ignore-macro=CLANG_TOOL_EXTRA_BUILD")
-  endif()
+option(LIBCLANG_INCLUDE_CLANG_TOOLS_EXTRA
+  "Include code from clang-tools-extra in libclang." OFF)
+
+if (LIBCLANG_INCLUDE_CLANG_TOOLS_EXTRA)
+  if (TARGET clangTidyPlugin)
+    add_definitions(-DCLANG_TOOL_EXTRA_BUILD)
+    list(APPEND LIBS clangTidyPlugin)
+    list(APPEND LIBS clangIncludeFixerPlugin)
+    if(LLVM_ENABLE_MODULES)
+      list(APPEND LLVM_COMPILE_FLAGS "-fmodules-ignore-macro=CLANG_TOOL_EXTRA_BUILD")
+    endif()
+  else ()
+    message(FATAL_ERROR "LIBCLANG_INCLUDE_CLANG_TOOLS_EXTRA needs clang-tools-extra in LLVM_ENABLE_PROJECTS")
+  endif ()
 endif ()
 
 find_library(DL_LIBRARY_PATH dl)

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

@@ -9,6 +9,11 @@ import("//llvm/version.gni")
 #   in the CMake build), so libclang is always a static library on linux
 # - the GN build doesn't have LIBCLANG_BUILD_STATIC
 
+declare_args() {
+  # Whether to include code from clang-tools-extra in libclang.
+  libclang_include_clang_tools_extra = false
+}
+
 libclang_target_type = "shared_library"
 if (host_os != "win" && host_os != "mac") {
   # ELF targets need -fPIC to build shared libs but they aren't on by default.
@@ -41,9 +46,7 @@ target(libclang_target_type, "libclang") {
 
   # FIXME: Once the GN build has a way to select which bits to build,
   # only include this dependency if clang-tools-extra is part of the build.
-  # FIXME: libclang depending on anything in clang-tools-extra seems like
-  # a layering violation.
-  if (true) {
+  if (libclang_include_clang_tools_extra) {
     defines += [ "CLANG_TOOL_EXTRA_BUILD" ]
     deps += [
       "//clang-tools-extra/clang-include-fixer/plugin",