A constraint like `Parameter == SomethingConcrete` means references to
`Parameter` in that context behave like `SomethingConcrete`, including for name
resolution. Handling the parameter as just a parameter type means that it won't
find any non-protocol nested types (i.e. things other than associated types and
protocol typealiases are invisible).
Fixes rdar://problem/42136457 and SR-8240.
