For Swift 3 / 4:
Deprecate the spelling "ImplicitlyUnwrappedOptional", emitting a warning
and suggesting "!" in places where they are allowed according to
SE-0054.
In places where SE-0054 disallowed IUOs but we continued to accept them
in previous compilers, emit a warning suggesting "Optional" or "?" as
an alternative depending on context and treat the IUO as an Optional,
noting this in the diagnostic.
For Swift 5:
Treat "ImplicitlyUnwrappedOptional" as an error, suggesting
"!" in places where they are allowed by SE-0054.
In places where SE-0054 disallowed IUOs, emit an error suggestion
"Optional" or "?" as an alternative depending on context.
Fix a couple more places where we should not allow IUOs and add
additional tests for these as well as parallel tests for the long
spelling ImplicitlyUnwrappedOptional.
We were allowing them in parens in some contexts, but shouldn't have.
Also added tests for function types and tuple return types to ensure
we're not allowing them in these places.
SE-0054 specified that the use of implicitly unwrapped optionals was
limited to just a few places, but the implementation at the time did not
completely ban the other uses. This is another attempt to do so, but
it's only on for compilations in Swift 5 mode and later.
For earlier versions, we fall back on the existing implementation.
Fixes: rdar://problem/27707015
