[clangd] Make AST-based signals available to runWithPreamble.

Many useful signals can be derived from a valid AST which is regularly updated by
the ASTWorker. `runWithPreamble` does not have access to the ParsedAST
but it can be provided access to some signals derived from a (possibly
stale) AST.

Differential Revision: https://reviews.llvm.org/D94424