[clangd] Populate #include insertions as additional edits in completion items.

Summary:
o Remove IncludeInsertion LSP command.
o Populate include insertion edits synchromously in completion items.
o Share the code completion compiler instance and precompiled preamble to get existing inclusions in main file.
o Include insertion logic lives only in CodeComplete now.
o Use tooling::HeaderIncludes for inserting new includes.
o Refactored tests.

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: klimek, ilya-biryukov, MaskRay, jkorous, cfe-commits

Differential Revision: https://reviews.llvm.org/D46497

llvm-svn: 332363