[clangd] Avoid indexing decls associated with friend decls.

Summary:
These decls are sometime used as the canonical declarations (e.g. for go-to-def),
which seems to be bad.

- friend decls that are not definitions should be ignored for indexing purposes
- this means they should never be selected as canonical decl
- if the friend decl is the only decl, then the symbol should not be indexed

Reviewers: sammccall

Reviewed By: sammccall

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

Differential Revision: https://reviews.llvm.org/D47623

llvm-svn: 333885