stdlib: use existing decodeCString for initializing a String with a null-terminated C string.

And remove the now unused _SentinelCollection utility.

Author: eeckstein

stdlib/public/core/CMakeLists.txt

@@ -108,7 +108,6 @@ set(SWIFTLIB_ESSENTIAL
   Runtime.swift.gyb
   RuntimeFunctionCounters.swift
   SipHash.swift
-  SentinelCollection.swift
   Sequence.swift
   SequenceAlgorithms.swift
   SequenceWrapper.swift

stdlib/public/core/GroupInfo.json

@@ -63,7 +63,6 @@
     "Repeat.swift",
     "Sort.swift",
     "Range.swift",
-    "SentinelCollection.swift",
     "ClosedRange.swift",
     "CollectionOfOne.swift",
     "HeapBuffer.swift",

stdlib/public/core/SentinelCollection.swift

@@ -231,11 +231,8 @@ extension String {
     decodingCString nullTerminatedCodeUnits: UnsafePointer<Encoding.CodeUnit>,
     as sourceEncoding: Encoding.Type) {
 
-    let codeUnits = _SentinelCollection(
-      UnsafeBufferPointer(_unboundedStartingAt: nullTerminatedCodeUnits),
-      until: _IsZero()
-    )
-    self.init(decoding: codeUnits, as: sourceEncoding)
+    self = String.decodeCString(
+      nullTerminatedCodeUnits, as: sourceEncoding)!.result
   }
 
   /// Calls the given closure with a pointer to the contents of the string,