[clangd] Fixes for #include insertion.

Summary:
o Avoid inserting a header include into the header itself.
o Avoid inserting non-header files (by not indexing symbols in main
files at all).
o Canonicalize include paths for symbols in dynamic index.

Reviewers: sammccall, ilya-biryukov

Reviewed By: ilya-biryukov

Subscribers: klimek, jkorous-apple, cfe-commits

Differential Revision: https://reviews.llvm.org/D43462

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@325523 91177308-0d34-0410-b5e6-96231b3b80d8
9 files changed