[swift-evolution] Change default compiler fix for not-unwrapped Optional from ! To ?

[Robert Bennett]

When writing Swift code, it is not uncommon to forget to unwrap optionals. The compiler will offer a fixit, telling you you must insert either a ? or a !. However, when you accept the fixit, ! is inserted (at least, it is for me in Xcode 8.3).

Ideally the fixit would default to ? because this is the preferred option; ! is often a sign of non-Swifty code and does not interact well with idiomatic Swift constructs such as if-let-(as?), guard-let-(as?)-else, etc. Also I think it’s safe to say that fixits should not err on the side of crashing at runtime.