[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