[clangd] Strip /showIncludes in clangd compile commands
In command lines with /showIncludes, clangd would output includes to stdout, breaking clients.
Summary: Fixes https://github.com/clangd/clangd/issues/322.
Reviewers: sammccall, kadircet
Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, usaxena95, cfe-commits
Tags: #clang
Differential Revision: https://reviews.llvm.org/D78836