When realizing a type like Foo<A>.Bar, we have to account for the
possibility that Bar is defined in a constrained extension of Foo,
and has generic requirements beyond those that Foo itself places
on 'A'.
Previously we only handled this for types referenced from the
constraint system as part of openUnboundGenericType(), so we were
allowing invalid types through in type context.
Add the right checking to applyGenericArguments() to close the
hole. Note that the old code path still exists in the constraint
solver; it is used for member accesses on metatype bases only.
Fixes <https://bugs.swift.org/browse/SR-10466>.
If generic parameter associated with missing conformance comes
from different context diagnose the problem as "referencing" a
specific declaration from affected type.
Instead of simply pointing out which type had conformance failures,
let's use affected declaration instead, which makes diagnostics much
richer e.g.
```
'List<[S], S.Id>' requires that 'S.Id' conform to 'Hashable'
```
versus
```
initializer 'init(_🆔)' requires that 'E' conform to 'Hashable' [with 'E' = 'S.Id']
```
Since latter message uses information about declaration, it can also
point to it in the source. That makes is much easier to understand when
problem is related to overloaded (function) declarations.
If solver encounters a typealias inside of constrainted extension
make sure to add its requirements to the constraint system, otherwise
it might produce invalid solutions.
