[clangd] Introduce client state invalidation

Clangd can invalidate client state of features like semantic higlighting
without client explicitly triggering, for example after a preamble build
caused by an onSave notification on a different file.

This patch introduces a mechanism to let client know of such actions,
and also calls the workspace/semanticTokens/refresh request to
demonstrate the situation after each preamble build.

Fixes https://github.com/clangd/clangd/issues/699.

Differential Revision: https://reviews.llvm.org/D97548