`long` is imported as `Int32` on LLP64, and `Int` on LP64. Use
`intptr_t` instead which is always imported as `Int`. This fixes the
clang-importer.objc_missing_designated_init test.
Objective-C allows this in "class extensions" (nameless categories);
what's worse is that it's actually useful-ish sometimes: when you want
to put a particular initializer in an explicit submodule, or only
expose it to the current target.
rdar://problem/37173549
(which can happen if an imported class has un-importable initializers)
Our initializer model guarantees that it's safe to inherit convenience
initializers when a subclass has implemented all designated
initializers, since each convenience initializer will be implemented
by calling one of the designated initializers. If one of the
designated initializers /can't/ be implemented in Swift, however,
then inheriting the convenience initializer would not be safe.
This is potentially a source-breaking change, so the importer will
only actually record that it failed to import something in when
compiling in Swift 4 mode.
rdar://problem/31563662
