[clangd] Handle multiple callbacks from Sema's completion

Summary:
When parser backtracks, we might receive multiple code completion
callbacks.
Previously we had a failing assertion there, now we take first results
and hope they are good enough.

Reviewers: sammccall

Reviewed By: sammccall

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

Differential Revision: https://reviews.llvm.org/D44567