In Swift 5.10 if you wrote `extension Foo {}` for some protocol Foo,
the extension would always re-use the generic signature of Foo, which
is <Self where Self: Foo>. In Swift 6 this no longer works because Foo
might be ~Copyable, in which case `extension Foo {}` adds default
requirements, so we changed GenericSignatureRequest to just always
build a new signature if we're given an extension.
However, to avoid a request cycle with a code example that really should
have never worked at all, I'm re-introducing the hack for re-using the
signature.
Fixes rdar://problem/129540617.
