Each candidate with incorrect labels (but everything else lined up)
gets a note on its declarationm which says what is expected and what
has been given.
This enables additional tests for the ClangImporter. This found a
missing piece in the `-enable-objc-interop` work that was done
previously. Address that and enable the tests. There are now the
following failing tests on Linux:
* sdk - depends on Foundation (not hermetic, see SR-7572)
* mixed-nsuinteger - depends on Foundation (not hermetic, see SR-7572)
* import-mixed-with-header-twice - requires apple/swift PR#16022
* can_import_objc_idempotent - requires apple/swift PR#16022
* objc_protocol_renaming - requires apple/swift PR#16022
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
