[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

llvm-svn: 325523