Direct lookup relied in primary file checking to have filled in the
protocol type stored in the ImplementsAttr. This was already wrong
with multi-file test cases in non-WMO mode, and crashed in the
ASTPrinter if printing a declaration in a non-primary file.
I don't have a standalone test case that is independent of my
upcoming ASTPrinter changes, but this is a nice cleanup regardless.
7499c222ee