Remove no-longer-used _Incrementable protocol (#12251)

Author: airspeedswift

stdlib/public/core/CMakeLists.txt

@@ -72,7 +72,6 @@ set(SWIFTLIB_ESSENTIAL
   HeapBuffer.swift
   ICU.swift
   ImplicitlyUnwrappedOptional.swift
-  Index.swift
   Indices.swift.gyb
   InputStream.swift
   IntegerParsing.swift

stdlib/public/core/GroupInfo.json

@@ -59,7 +59,6 @@
     "Sequence.swift",
     "SequenceAlgorithms.swift",
     "RangeReplaceableCollection.swift",
-    "Index.swift",
     "Indices.swift",
     "Existential.swift",
     "WriteBackMutableSlice.swift",

stdlib/public/core/Index.swift

@@ -16,7 +16,6 @@
 
 // RUN: %target-swift-ide-test -print-module -module-to-print=Swift -source-filename %s -print-interface -skip-underscored-stdlib-protocols > %t-prot.txt
 // RUN: %FileCheck -check-prefix=CHECK-UNDERSCORED-PROT %s < %t-prot.txt
-// CHECK-UNDERSCORED-PROT: public protocol _Incrementable
 // CHECK-UNDERSCORED-PROT: public protocol _SequenceWrapper
 // CHECK-UNDERSCORED-PROT-NOT: protocol _