Force tests to build with -swift-version 3.

Author: tkremenek

Tests/Functional/lit.cfg

@@ -37,8 +37,10 @@ def _getenv(name):
     return value
 
 built_products_dir = _getenv('BUILT_PRODUCTS_DIR')
+# Force tests to build with -swift-version 3 for now.
 swift_exec = [
     _getenv('SWIFT_EXEC'),
+    '-swift-version', '3',
     '-Xlinker', '-rpath',
     '-Xlinker', built_products_dir,
     '-L', built_products_dir,