[clangd] Correct setting ignoreWarnings in CodeCompletion.

[hokein]

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.

[ilya-biryukov]

LGTM with a minor nit.

clangd/CodeComplete.cpp

705

Could we add a comment that this should be done before calling prepareCompilerInstance?
I'm afraid this can be accidentally reordered when changing the code.

[hokein]

• add a comment
• find one more incorrect usage, and fix it.

[hokein]

clangd/CodeComplete.cpp

705

Yeah! Done.

clangd/CodeComplete.cpp

auto Bounds =
         ComputePreambleBounds(*CI->getLangOpts(), ContentsBuffer.get(), 0);
     // FIXME(ibiryukov): Remove this call to CanReuse() after we'll fix
     // clients relying on getting stats for preamble files during code
     // completion.
     // Note that results of CanReuse() are ignored, see the comment above.
     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;
 
   auto &FrontendOpts = Clang->getFrontendOpts();
   FrontendOpts.SkipFunctionBodies = true;
   FrontendOpts.CodeCompleteOpts = Options;
   FrontendOpts.CodeCompletionAt.FileName = Input.FileName;

clangd/Headers.cpp

return llvm::make_error<llvm::StringError>(
         "Failed to create a compiler instance for " + File,
         llvm::inconvertibleErrorCode());
   CI->getFrontendOpts().DisableFree = false;
   // Parse the main file to get all existing #includes in the file, and then we
   // can make sure the same header (even with different include path) is not
   // added more than once.
   CI->getPreprocessorOpts().SingleFileParseMode = true;
 
+  // The diagnostic options must be set before creating a CompilerInstance.
+  CI->getDiagnosticOpts().IgnoreWarnings = true;
   auto Clang = prepareCompilerInstance(
       std::move(CI), /*Preamble=*/nullptr,
       llvm::MemoryBuffer::getMemBuffer(Code, File),
       std::make_shared<PCHContainerOperations>(), FS, IgnoreDiags);
-  auto &DiagOpts = Clang->getDiagnosticOpts();
-  DiagOpts.IgnoreWarnings = true;
 
   if (Clang->getFrontendOpts().Inputs.empty())
     return llvm::make_error<llvm::StringError>(
         "Empty frontend action inputs empty for file " + File,
         llvm::inconvertibleErrorCode());
   PreprocessOnlyAction Action;
   if (!Action.BeginSourceFile(*Clang, Clang->getFrontendOpts().Inputs[0]))
     return llvm::make_error<llvm::StringError>(