[clangd] Set completion options per-request.

Summary:
Previously, completion options were set per ClangdServer instance.
It will allow to change completion preferences during the lifetime
of a single ClangdServer instance.

Also rewrote ClangdCompletionTest.CompletionOptions to reuse single
ClangdServer instance, the test now runs 2x faster on my machine.

Reviewers: sammccall, ioeric, hokein

Reviewed By: sammccall, ioeric

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D40654

llvm-svn: 319753