[clangd] Cache config files for 5 seconds, without revalidating with stat.

Summary:
This is motivated by:
 - code completion: nice to do no i/o on the request path
 - background index: deciding whether to enqueue each file would stat the config
   file thousands of times in quick succession.

Currently it's applied uniformly to all requests though.

This gives up on performing stat() outside the lock, all this achieves is
letting multiple threads stat concurrently (and thus finish without contention
for nonexistent files).
The ability to finish without IO (just mutex lock + integer check) should
outweigh this, and is less sensitive to platform IO characteristics.

Reviewers: kadircet

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D83755