[clangd] Enable SuggestMissingIncludes by default.

Summary: This seems to work stably now. Turn on by default.

Reviewers: kadircet

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D58772

llvm-svn: 355200