Only declarations in the same module as synthesized extension's target
are placed within a synthesized extension. We should thus not add
"::SYNTHESIZED::" to the USR if the given declaration is in a different
and.
As an example, `Foundation` adds a method `components(separatedBy:)` to
`String` through an extension on `StringProtocol`. But since it is
within `Foundation` and not `Swift` it will *not* be in a synthesized
extension of `String` or `StringProtocol`. So it should not have
"::SYNTHESIZED::" added and should also not being in the `String` group.
Resolves rdar://71355632.
