[clangd] Catch more symbols in SymbolCollector.

Summary:
We currently only collect external-linkage symbols in the collector,
which results in missing some typical symbols (like no-linkage type alias symbols).

This patch relaxes the constraint a bit to allow collecting more symbols.

Reviewers: ilya-biryukov

Reviewed By: ilya-biryukov

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D41759

llvm-svn: 322067