[clangd] New ParsingCallback for semantics changes
Previously notification of the Server about semantic happened strictly
before notification of the AST thread.
Hence a racy Server could make a request (like semantic tokens) after
the notification, with the assumption that it'll be served fresh
content. But it wasn't true if AST thread wasn't notified about the
This change reverses the order of those notifications to prevent racy
Differential Revision: https://reviews.llvm.org/D102761