Revert removal of tidy plugin support from libclang

[yvvan]

Summary

libclang has nothing to do with clangd so I don't see why having the last one is the reason to remove features from libclang. Especially without reviews.

[yvvan]

I generated the wrong diff. This is the proper one.

[nik]

+1 as this seems to fix the regression.

[yvvan]

We can also add an extra variable for it if you care about build time

[bkramer]

I'd be interested in hearing how this is used. I added this feature as an experiment a while back but it simply didn't work as I envisioned it to. Some checks do work but the overall latency makes it unusable in an IDE setting. People repeatedly asked me to remove it because it slows down builds while not adding value.

[yvvan]

In D55415#1323120, @bkramer wrote:

I'd be interested in hearing how this is used. I added this feature as an experiment a while back but it simply didn't work as I envisioned it to. Some checks do work but the overall latency makes it unusable in an IDE setting. People repeatedly asked me to remove it because it slows down builds while not adding value.

We use it with Qt Creator and you can see that we make additions/fixes in libclang from time to time.
With tidy checks selected it is slow indeed but there's always a choice whether to enable checks or not. And if you have very few checks enabled it performs quite fine. And we are still using libclang because we already have infrastructure on top of it which can't be just replaced with clangd (at least it requires quite some time).

[bkramer]

I think that's a fair point for bringing it back for now. It's not supported though and we will get rid of it eventually.

tools/libclang/CIndex.cpp

     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;
+
+// This anchor is used to force the linker to link the clang-include-fixer
+// plugin.
+extern volatile int ClangIncludeFixerPluginAnchorSource;
+static int LLVM_ATTRIBUTE_UNUSED ClangIncludeFixerPluginAnchorDestination =
+    ClangIncludeFixerPluginAnchorSource;
+#endif

tools/libclang/CMakeLists.txt

   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()
+endif ()
+
 find_library(DL_LIBRARY_PATH dl)
 if (DL_LIBRARY_PATH)
   list(APPEND LIBS dl)