[clangd] Add -header-insertion=never flag to disable include insertion in code completion

Summary: One clear use case: use with an editor that reacts poorly to edits above the cursor.

Reviewers: ioeric

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D60409

llvm-svn: 358075