swift234

Remove the fake _CocoaStringType protocol, NFC.

Adding a conformance to Foundation doesn't work because NSString can
be used without loading Foundation. debugDescription is one example of
this.

The only value we derive from the _CocoaStringType is its name, which
makes some APIs more readable. Adding a type safe wrapper around it
serves no purpose since we're almost always immediately casting back
and forth from an 'id'. This was previously done with unsafeBitCast,
which should be avoided unless we really need to reinterpret a bit
pattern.