In order to allow supporting `__strong` (and `__weak`) struct fields,
some parts of the ClangImporter needs to understand them. The changes in
this commit allows the type importer to allow the already supported
`__unsafe_unretained` struct fields, but still reject the `__strong` and
`__weak` fields. Later changes will add support for bridging `__strong`
and `__weak` fields.
All the code should be equivalent to the previous code, and since all
the structs with non-trivial copy/destroy are completely discarded, the
code should not even be hit in any case.
The included modifications in the tests check that the error and the
diagnostics note are produced correctly.
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.
