Sema: Add a test case for unsupported existential type in 'where' clause
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 c46eb22fcd78acf5ed2bdbf1c36540dc608ca2b6
Author: Slava Pestov <spestov@apple.com>
Date:   Fri Jul 24 23:54:22 2020 -0400

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

xwu/swift,tkremenek/swift,atrick/swift,xwu/swift,parkera/swift,gregomni/swift,glessard/swift,rudkx/swift,xwu/swift,xwu/swift,ahoppen/swift,xwu/swift,hooman/swift,rudkx/swift,atrick/swift,tkremenek/swift,atrick/swift,hooman/swift,parkera/swift,xwu/swift,rudkx/swift,xwu/swift,parkera/swift,ahoppen/swift,gregomni/swift,glessard/swift,apple/swift,rudkx/swift,roambotics/swift,gregomni/swift,tkremenek/swift,JGiola/swift,parkera/swift,JGiola/swift,ahoppen/swift,glessard/swift,apple/swift,hooman/swift,apple/swift,benlangmuir/swift,tkremenek/swift,glessard/swift,hooman/swift,glessard/swift,JGiola/swift,glessard/swift,parkera/swift,tkremenek/swift,tkremenek/swift,rudkx/swift,JGiola/swift,apple/swift,parkera/swift,gregomni/swift,benlangmuir/swift,benlangmuir/swift,hooman/swift,roambotics/swift,benlangmuir/swift,roambotics/swift,rudkx/swift,parkera/swift,roambotics/swift,apple/swift,parkera/swift,benlangmuir/swift,hooman/swift,roambotics/swift,ahoppen/swift,JGiola/swift,gregomni/swift,JGiola/swift,apple/swift,benlangmuir/swift,atrick/swift,atrick/swift,tkremenek/swift,atrick/swift,gregomni/swift,hooman/swift,ahoppen/swift,ahoppen/swift,roambotics/swift