[clangd] Correct setting ignoreWarnings in CodeCompletion.

Summary:
We should set the flag before creating ComplierInstance -- when
CopmilerInstance gets initialized, it also initializes the DiagnosticsEngine
using the DiagnosticOptions.

This was hidden deeply -- as clang suppresses all diagnostics when we
hit the code-completion (but internally it does do unnecessary analysis stuff).

As a bonus point, this fix will optmize the completion speed -- clang won't do
any analysis (e.g. -Wunreachable-code, -Wthread-safety-analysisi) at all internally.

Reviewers: ilya-biryukov

Reviewed By: ilya-biryukov

Subscribers: klimek, jkorous-apple, ioeric, cfe-commits

Differential Revision: https://reviews.llvm.org/D43569

llvm-svn: 325779
diff --git a/clang-tools-extra/clangd/CodeComplete.cpp b/clang-tools-extra/clangd/CodeComplete.cpp
index d582adc..74ea89d 100644
--- a/clang-tools-extra/clangd/CodeComplete.cpp
+++ b/clang-tools-extra/clangd/CodeComplete.cpp
@@ -696,11 +696,11 @@
     Input.Preamble->CanReuse(*CI, ContentsBuffer.get(), Bounds,
                              Input.VFS.get());
   }
+  // The diagnostic options must be set before creating a CompilerInstance.
+  CI->getDiagnosticOpts().IgnoreWarnings = true;
   auto Clang = prepareCompilerInstance(
       std::move(CI), Input.Preamble, std::move(ContentsBuffer),
       std::move(Input.PCHs), std::move(Input.VFS), DummyDiagsConsumer);
-  auto &DiagOpts = Clang->getDiagnosticOpts();
-  DiagOpts.IgnoreWarnings = true;
 
   // Disable typo correction in Sema.
   Clang->getLangOpts().SpellChecking = false;