[clangd] Fix some references missing in dynamic index.

Summary:
Previously, SymbolCollector postfilters all references at the end to
find all references of interesting symbols.
It was incorrect when indxing main AST where we don't see locations
of symbol declarations and definitions in the main AST (as those are in
preamble AST).

The fix is to do earily check during collecting references.

Reviewers: sammccall

Subscribers: ilya-biryukov, ioeric, MaskRay, jkorous, arphaman, kadircet, cfe-commits

Differential Revision: https://reviews.llvm.org/D53273

llvm-svn: 344507