[clangd] Introduce pullDiags endpoint

Implement initial support for pull-based diagnostics in ClangdServer.
This is planned for LSP 3.17, and initial proposal is in
https://github.com/microsoft/vscode-languageserver-node/blob/d15eb0671e0947d14e3548d13754104ee13aa56d/protocol/src/common/proposed.diagnostic.ts#L111.

We chose to serve the requests only when clangd has a fresh preamble
available. In case of a stale preamble we just drop the request on the
floor.

This patch doesn't plumb this to LSP layer yet, as pullDiags is still a
proposal with only an implementation in vscode.

Differential Revision: https://reviews.llvm.org/D98623