freestanding tests fail to build on Apple platforms

Apple platforms require reflection to be enabled in the standard library. Re-enable that in mixin_stdlib_minimal.

rdar://135878323

Author: ian-twilightcoder

utils/build-presets.ini

@@ -2771,7 +2771,7 @@ swift-enable-dispatch=0
 swift-implicit-concurrency-import=0
 swift-stdlib-support-back-deployment=0
 swift-enable-backtracing=0
-swift-enable-reflection=0
+swift-enable-reflection=1
 swift-stdlib-reflection-metadata=debugger-only
 swift-stdlib-stable-abi=0
 swift-stdlib-has-dladdr=0