[clangd] Shutdown cleanly on signals.

Summary:
This avoids leaking PCH files if editors don't use the LSP shutdown protocol.

This is one fix for https://github.com/clangd/clangd/issues/209
(Though I think we should *also* be unlinking the files)

Reviewers: kadircet, jfb

Subscribers: mgorny, ilya-biryukov, MaskRay, jkorous, arphaman, jfb, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D70684