[DiagnosticsQoI] Bail on nil literal diagnostics if the source is Optional

[CodaFi]

A tiny change to get back some QoI because it isn't sufficient to just observe that the initializer of a pattern binding failed a contextual conversion from nil. If we fire the existing diagnostic, it would pile on optional sugar and never tell the user what actually went wrong.

// Before
var _: Array? = nil // nil cannot initialize specified type 'Array?', add '?' to form the optional type 'Array??'
var _: Array?? = nil // nil cannot initialize specified type 'Array??', add '?' to form the optional type 'Array???'
var _: Array??? = nil // nil cannot initialize specified type 'Array???', add '?' to form the optional type 'Array????'
// ...

// After
var _: Array? = nil // generic parameter 'Element' could not be inferred

"Reported" by a Redditor.

[CodaFi]

@swift-ci please smoke test

[xedin]

Makes sense, thanks @CodaFi!

[CodaFi]

⛵️

[krypt-lynx]

Wow, it was fast.