[stdlib] Make a generic more idiomatic

NFC

Author: Dave Abrahams

stdlib/public/core/Unicode.swift

@@ -591,12 +591,10 @@ public func transcode<Input, InputEncoding, OutputEncoding>(
 ///
 /// Returns the index of the first unhandled code unit and the UTF-8 data
 /// that was encoded.
-internal func _transcodeSomeUTF16AsUTF8<Input>(
+internal func _transcodeSomeUTF16AsUTF8<Input : Collection>(
   _ input: Input, _ startIndex: Input.Index
 ) -> (Input.Index, _StringCore._UTF8Chunk)
-  where
-  Input : Collection,
-  Input.Iterator.Element == UInt16 {
+  where Input.Iterator.Element == UInt16 {
 
   typealias _UTF8Chunk = _StringCore._UTF8Chunk