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.
This was causing the importer to complain about not being able to map
'dispatch_sync' to 'DispatchQueue.sync(self:execute:)', but only when
building the PCM for our mock Dispatch. That meant that it depended on
the test order whether we'd see issues---if the first test to build
the module didn't check for warnings in other files, it would slip by.
This was usually blocks_parse.swift.
rdar://problem/30475805
