Bump default version in build script.

Author: tkremenek

utils/build-script

@@ -2044,7 +2044,7 @@ iterations with -O",
         "--swift-user-visible-version",
         help="User-visible version of the embedded Swift compiler",
         type=arguments.type.swift_compiler_version,
-        default="3.1",
+        default="4.0",
         metavar="MAJOR.MINOR")
 
     parser.add_argument(