[clangd] don't insert new includes if either original header or canonical...

[clangd] don't insert new includes if either original header or canonical header is already included.

Summary:
Changes:
o Store both the original header and the canonical header in LSP command.
o Also check that both original and canonical headers are not already included
by comparing both resolved header path and written literal includes.

This addresses the use case where private IWYU pragma is defined in a private
header while it would still be preferrable to include the private header, in the
internal implementation file. If we have seen that the priviate header is already
included, we don't try to insert the canonical include.

Reviewers: sammccall

Reviewed By: sammccall

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

Differential Revision: https://reviews.llvm.org/D43510

llvm-svn: 326070