[clangd] downgrade missing-includes diagnostic to Information level

In practice, a Warning on every occurrence is very unpopular, even on a codebase
with clear rules about direct inclusion & moderately good compliance.

This change has various practical effects (in vscode for concreteness):
  - makes the diagnostic decoration less striking (blue vs yellow)
  - makes these diagnostics visually distinct from others when reading
  - causes these diagnostics to sort last in the "problems" view
  - allows these diagnostics to be easily filtered from the "problems" view

Differential Revision: https://reviews.llvm.org/D149912