[clangd] Publish diagnostics with stale preambles

This patch achieves this by building an AST and invoking main file
callbacks on each update, in addition to preamble updates.

It means we might have some extra AST builds now (e.g. if an update was
with a stale preamble and there were no reads on it, we would only build
an AST once we had the fresh preamble. Now we'll build 2, once with the
stale preamble and another with the fresh one, but we'll have one more
diagnostics cycle in between.).

This patch preserves forward progress of diagnostics by always using the
latest main file contents when emitting diagnostics after preamble
builds. It also guarantees eventual consistency:
- if an update doesn't invalidate preamble, we'll emit diagnostics with
  fresh preamble already.
- if an update invalidates preamble, we'll first emit diagnostics with
  stale contents, and then once the preamble build finishes it'll emit
  diagnostics (as preamble has changed) with newest version.

This has implications on parsing callbacks, as previously onMainAST
callback was called at most once, now it can be called up to 2 times.
All of the existing clients can already deal with callback firing
multiple times.

Differential Revision: https://reviews.llvm.org/D144456