[clang] Add `forceReload` clangd extension to 'textDocument/didChange'

Summary:
- This option forces a preamble rebuild to handle the odd case
  of a missing header file being added

Reviewers: sammccall

Subscribers: ilya-biryukov, javed.absar, MaskRay, jkorous, arphaman, jfb, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D73916
diff --git a/clang-tools-extra/clangd/ClangdLSPServer.cpp b/clang-tools-extra/clangd/ClangdLSPServer.cpp
index 880359f..93609a8 100644
--- a/clang-tools-extra/clangd/ClangdLSPServer.cpp
+++ b/clang-tools-extra/clangd/ClangdLSPServer.cpp
@@ -647,7 +647,7 @@
     return;
   }
 
-  Server->addDocument(File, *Contents, WantDiags);
+  Server->addDocument(File, *Contents, WantDiags, Params.forceRebuild);
 }
 
 void ClangdLSPServer::onFileEvent(const DidChangeWatchedFilesParams &Params) {