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
Sam McCall