This fixes a regression from the previous patch which got
rid of PrintOptions::StripDynamicSelf.
When printing protocol declarations with a BaseType set in
PrintOptions, we can end up with a DynamicSelfType wrapping
a non-class type, if the protocol requirement returned
Self.
Note that this changes the diagnostic for missing protocol
requirements slightly; we used to sometimes refer to 'Self'
even if the conforming type is not a class, which is not
accepted by the type checker anyway. I believe the new
diagnostics are more correct.
This was causing us to emit diagnostics talking about τ_m_n, which is
not helpful.
Now that generic function types print sanely, print them in a few
places where we were previously printing PolymorphicFunctionTypes.
* If current context is extension of non-final class, don't suggest stub
* If current context is extension of final class, insert 'convenience'
* If current context is direct definition of non-final class, insert 'required'
To each note with a protocol requirement that is not met, a fixit is
added that inserts a stub for the requirement at the start of the
adoptee's declaration.
