SI-8407 "symbol not found" in Scaladoc use cases: warning only

The refactoring in aedec1980 mistakenly used the non-silent
typer to typecheck the use cases. It temptingly had the desired
type (`ScalaDocAnalyzer`), but sadly the wrong properties (it will
report errors, rather than buffer.)

This meant that "symbol not found" errors in use cases would prevent
Scaladoc generation.

This commit introduces a little downcast on the silent typer.
A more principled approach would be to go through the rigmarole
of the virtual class pattern for `Analzyer.Typer`, but that will
have to remain work for another day.
Merge pull request #3627 from retronym/ticket/8407

SI-8407 "symbol not found" in Scaladoc use cases: warning only