Support partially qualified names in workspace/symbol

[HighCommander4]

I originally filed this as clangd/vscode-clangd#31 (it was transferred into the vscode-clangd repo after filing). That ticket has been closed, but the issue is only partially fixed.

Specifically, in that ticket, a vscode-specific issue that led to workspace/symbol queries with a fully qualified name as a search string not returning results, was fixed.

There is a remaining issue on the server side, that workspace/symbol queries with partially qualified names do not return results. For example, searching for Class::Method should match ns::Class::Method or ns1::ns2::Class::Method if such entities exist.

[kadircet]

That definitely makes sense to me, send out https://reviews.llvm.org/D88814. This might degrade the search performance for top-level but not fully-qualified searches. e.g. clang:: vs ::clang. But I think providing results for sub-scopes like clangd:: is definitely worth it.

I wonder what will others think tho. Hence did not assign anyone for review yet :D

[kadircet]

To summarize an offline discussion and give a bit more details:

• An unqualified lookup (e.g. Foo) or a fully-qualified lookup (e.g. ::Foo) isn't going to be affected by this change.
• Qualified lookups of the form foo::bar::Baz are going to be a lookup in all scopes, with symbols from ::foo::bar being boosted and any results that doesn't have foo::bar:: as a "sub-scope" will be filtered.