Always ask Cocoa for terminated char*s

(cherry picked from commit b7ce22ba1525dccd47f4095a9bf8a640b238a5fa)

Author: Catfish-Man

stdlib/public/core/StringBridge.swift

@@ -351,7 +351,9 @@ internal func _bridgeTagged(
 private func _NSStringASCIIPointer(_ str: _StringSelectorHolder) -> UnsafePointer<UInt8>? {
  // TODO(String bridging): Is there a better interface here? Ideally we'd be
   // able to ask for UTF8 rather than just ASCII
-  return str._fastCStringContents(0)?._asUInt8
+  //TODO(String bridging): Unconditionally asking for nul-terminated contents is
+  // overly conservative and hurts perf with some NSStrings
+  return str._fastCStringContents(1)?._asUInt8
 }
 
 @_effects(readonly) // @opaque