Fix sub-scalar index distances in foreign UTF8 views

Author: Catfish-Man

stdlib/public/core/StringBridge.swift

@@ -210,6 +210,7 @@ internal func _cocoaStringUTF8Count(
   _ target: _CocoaString,
   range: Range<Int>
 ) -> Int? {
+  if range.isEmpty { return 0 }
   return _NSStringUTF8Count(_objc(target), range: range)
 }
 

stdlib/public/core/StringUTF8View.swift

@@ -506,7 +506,7 @@ extension String.UTF8View {
       // _cocoaStringUTF8Count gave us the scalar aligned count, but we still
       // need to compensate for sub-scalar indexing, e.g. if `i` is in the
       // middle of a two-byte UTF8 scalar.
-      let refinedCount = count - (i.transcodedOffset + j.transcodedOffset)
+      let refinedCount = (count - i.transcodedOffset) + j.transcodedOffset
       _internalInvariant(refinedCount == _distance(from: i, to: j))
       return refinedCount
     }

test/stdlib/Inputs/NSSlowString/NSSlowString.m

@@ -31,7 +31,7 @@ - (NSUInteger)length {
     return self.stringHolder.length;
 }
 
-- (id)copy {
+- (id)copyWithZone:(NSZone *)unused {
 	return self;
 }