[libclang] Link clang-tidy plugin into libclang if present.

[bkramer]

Summary

This is a sad workaround for the lack of plugin support in libclang. Depends
on D17807, a tools-extra change that also contains the test case.

This is designed to be easy to remove again if libclang ever grows proper
plugin support.

[klimek]

lg

tools/libclang/CIndex.cpp

#endif
   OS << llvm::format("%7.4f] ", TR.getWallTime() - sBeginTR.getWallTime());
   OS << Msg << '\n';
 
   if (Trace) {
     llvm::sys::PrintStackTrace(OS);
     OS << "--------------------------------------------------\n";
   }
 }
+
+#ifdef CLANG_TOOL_EXTRA_BUILD
+// This anchor is used to force the linker to link the clang-tidy plugin.
+extern volatile int ClangTidyPluginAnchorSource;
+static int LLVM_ATTRIBUTE_UNUSED ClangTidyPluginAnchorDestination =
+    ClangTidyPluginAnchorSource;
+#endif

tools/libclang/CMakeLists.txt

set(LIBS
   clangSema
   clangTooling
 )
 
 if (CLANG_ENABLE_ARCMT)
   list(APPEND LIBS clangARCMigrate)
 endif ()
 
+if (CLANG_TOOL_EXTRA_BUILD)
+  add_definitions(-DCLANG_TOOL_EXTRA_BUILD)
+  list(APPEND LIBS clangTidyPlugin)
+endif ()
+
 find_library(DL_LIBRARY_PATH dl)
 if (DL_LIBRARY_PATH)
   list(APPEND LIBS dl)
 endif()
 
 option(LIBCLANG_BUILD_STATIC
   "Build libclang as a static library (in addition to a shared one)" OFF)