[clangd] Limit completion results.

Summary:
* truncate symbols from static/dynamic index to the limited number
(which would save lots of cost in constructing the merged symbols).
* add an CLI option allowing to limit the number of returned completion results.
(default to 100)

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: klimek, ilya-biryukov, jkorous-apple, ioeric, cfe-commits

Differential Revision: https://reviews.llvm.org/D42484

llvm-svn: 323408