Disable warnings when indexing as a standalone action.

Summary:
- we don't record the warnings at all
- we don't want to stop indexing if we hit error-limit due to warnings
- this allows some analyses to be skipped which can save some CPU

https://github.com/clangd/clangd/issues/24

Reviewers: hokein

Subscribers: ilya-biryukov, ioeric, jkorous, arphaman, kadircet, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D59935

llvm-svn: 357186