[clangd] Move the existing heder-source-switch implemenation out of clangdServer.

Summary: This is a NFC change.

Reviewers: kadircet

Reviewed By: kadircet

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D67960

llvm-svn: 372841