Build Swift-Testing with WMO

This tells build-script to build Swift-testing with WMO.
This results in a faster build products, but is also necessary for
configurations using the legacy swift driver, which would emit an
invalid swift interface in non-WMO builds.

Fixes: rdar://151357567

utils/swift_build_support/swift_build_support/products/swift_testing.py

@@ -112,6 +112,8 @@ class SwiftTestingCMakeShim(cmake_product.CMakeProduct):
 
         self.cmake_options.define('CMAKE_BUILD_TYPE', self.args.build_variant)
 
+        self.cmake_options.define('CMAKE_Swift_COMPILATION_MODE', 'wholemodule')
+
         # FIXME: If we build macros for the builder, specify the path.
         self.cmake_options.define('SwiftTesting_MACRO', 'NO')