[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