[clangd][Background] Make index validation logs verbose

These errors are non-harmful and should be transient. They either
imply:
- compilation database returned stale results for TUs and it'll be fixed once
  it's updated to match project state.
- a TUs dependencies has changed and some headers no longer exist. this should
  be fixed with the next indexing cycle.

In either case the user will have some stale symbols in their index until clangd
restarts and the underlying issue is resolved. On the downside these logs are
confusing users when there's another issue.

Differential Revision: https://reviews.llvm.org/D117792