Some APIs that expected a String now expect a Substring and vice
versa. To ease the transition, emit fix-its on conversion errors
between these types that the migrator can pick up.
When converting from Substring -> String, suggest wrapping in
`String.init`.
When converting from String -> Substring, suggest appending the
void subscript `[]`. (This isn't implemented yet so this is
hidden behind a flag).
This can possibly be generalized later when converting between
some sequence and its subsequence, such as Array and ArraySlice,
for example.
rdar://problem/31665649
rdar://problem/31666638
There is no implicit conversion between String and Substring, so
the migrator can offer fix-its to convert between the two by wrapping
in String.init or adding the void subscript [] accordingly. This
commit just adds the tests for migration for the time being.
rdar://problem/30928902
