[clangd] Document highlights for clangd

Summary: Implementation of Document Highlights Request as described in
LSP.

Contributed by William Enright (nebiroth).

Reviewers: malaperle, krasimir, bkramer, ilya-biryukov

Reviewed By: malaperle

Subscribers: mgrang, sammccall, klimek, ioeric, rwols, cfe-commits, arphaman, ilya-biryukov

Differential Revision: https://reviews.llvm.org/D38425

llvm-svn: 320474
diff --git a/clang-tools-extra/clangd/ProtocolHandlers.cpp b/clang-tools-extra/clangd/ProtocolHandlers.cpp
index ce3204b..9f6f335 100644
--- a/clang-tools-extra/clangd/ProtocolHandlers.cpp
+++ b/clang-tools-extra/clangd/ProtocolHandlers.cpp
@@ -72,4 +72,6 @@
   Register("textDocument/rename", &ProtocolCallbacks::onRename);
   Register("workspace/didChangeWatchedFiles", &ProtocolCallbacks::onFileEvent);
   Register("workspace/executeCommand", &ProtocolCallbacks::onCommand);
+  Register("textDocument/documentHighlight",
+           &ProtocolCallbacks::onDocumentHighlight);
 }