[clangd] Always use preamble (even stale) for code completion

Summary:
This improves performance of code completion, because we avoid stating
the files from the preamble and never attempt to parse the files
without using the preamble if it's provided.

However, the change comes at a cost of sometimes providing incorrect
results when doing code completion after making actually considerable
changes to the files used in the preamble or the preamble itself.
Eventually the preamble will get rebuilt and code completion will
be providing the correct results.

Reviewers: bkramer, sammccall, jkorous-apple

Reviewed By: sammccall

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D41991

llvm-svn: 322854