[clangd] Simplify "preferred" vs "definition" logic a bit in XRefs AST code.

Summary:
Now Preferred is always the canonical (first) decl, Definition is always the def
if available.

In practice the index was already forcing this behaviour anyway, so there's no
change. (Unless you weren't using this index, in which case this patch makes
textDocument/declaration and toggling work as expected).

Reviewers: kadircet

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D73369