Changes in shadowing behaviour by #15412 caused a property on a concrete type to no longer shadow a protocol property member, which created unintentional ambiguities in 4.2. This commit ensures we at least keep these cases unambiguous in Swift 5 under Swift 4 compatibility mode. This is intentionally narrow in order to best preserve source compatibility under Swift 4 mode by ensuring we don't introduce any new ambiguities.
Resolves SR-7425, SR-7940 & SR-8343.
