This looks like a nice improvement.
These tests are nice.
But we're changing Sema in this patch, I think we'd better to have these in clang, /test/CodeCompletion/call.cpp seems to be the place.
I'm a bit nervous to show all overloads regardlessly, I think we could do some smarter things like
2 seems quite critical and trivial to implement, maybe we can do it in this patch.
well actually now that I think about it, this is actually not that hard. possibly even easier than current version. changing the logic surface every candidate using the prefix, and do a post-filtering instead of trying to mock all the results.