Sema: Add a test case for unsupported existential type in 'where' clause

In Swift 5.3, we only checked the 'where' clause for unsupported
existential types if the declaration was itself generic. If the
declaration was only nested inside of another generic declaration,
the check was skipped.

This was fixed by the following refactoring but I didn't realize it at
the time:

commit c46eb22
Author: Slava Pestov <[email protected]>
Date:   Fri Jul 24 23:54:22 2020 -0400

    AST: Don't attach trailing where clause requirements to the GenericParamList

Author: slavapestov

test/type/protocol_types.swift

@@ -138,3 +138,8 @@ struct BadSubscript {
     set {}
   }
 }
+
+struct OuterGeneric<T> {
+  func contextuallyGenericMethod() where T == HasAssoc {}
+  // expected-error@-1 {{protocol 'HasAssoc' can only be used as a generic constraint because it has Self or associated type requirements}}
+}