[clangd] Wait for first preamble before code completion

Summary:
To avoid doing extra work of processing headers in the preamble
mutilple times in parallel.

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: javed.absar, ioeric, MaskRay, jkorous, cfe-commits

Differential Revision: https://reviews.llvm.org/D48940

llvm-svn: 336538