[clangd] Use -completion-style=bundled by default if signature help is available

Summary:
I didn't manage to find something nicer than optional<bool>, but at least I
found a sneakier comment.

Reviewers: kadircet

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64216

llvm-svn: 365356