[benchmark] Use String.split instead of String.components.

This allows the benchmarks to be built without the Foundation overlay being
built. I am currently prototyping having +0 parameters for all normal arguments
and have not gotten the Foundation overlay to work yet... so this commit will
let me get some initial numbers for the subset of the tests that do not depend
on Foundation.

rdar://34222540

Author: gottesmm

benchmark/utils/ArgParse.swift

@@ -55,16 +55,16 @@ public func parseArgs(_ validOptions: [String]? = nil)
       continue
     }
     // Attempt to split it into two components separated by an equals sign.
-    let components = arg.components(separatedBy: "=")
-    let optionName = components[0]
+    let components = arg.split(separator: "=")
+    let optionName = String(components[0])
     if validOptions != nil && !validOptions!.contains(optionName) {
       print("Invalid option: \(arg)")
       return nil
     }
     var optionVal : String
     switch components.count {
       case 1: optionVal = ""
-      case 2: optionVal = components[1]
+      case 2: optionVal = String(components[1])
       default:
       // If we do not have two components at this point, we can not have
       // an option switch. This is an invalid argument. Bail!