Collapse PrefixWhileBidirectionalCollection and de-gyb PrefixWhile.swift (#14910)

Author: airspeedswift

stdlib/public/core/CMakeLists.txt

@@ -95,7 +95,7 @@ set(SWIFTLIB_ESSENTIAL
   OutputStream.swift
   Pointer.swift
   Policy.swift
-  PrefixWhile.swift.gyb
+  PrefixWhile.swift
   Print.swift
   RandomAccessCollection.swift
   Range.swift