standalone minimal tests fail to build on Apple platforms

[ian-twilightcoder]

Apple platforms require reflection to be enabled in the standard library. Revert the commit that disabled reflection.

rdar://135878323

This reverts commit 8b189d9.

# Conflicts:
# stdlib/private/StdlibUnittest/CMakeLists.txt
# stdlib/private/StdlibUnittest/StdlibCoreExtras.swift
# stdlib/public/Platform/CMakeLists.txt
# utils/build-presets.ini

[ian-twilightcoder]

@kubamracek I see this was originally turned off in #39030 but I don't understand why

[ian-twilightcoder]

preset=stdlib_S_standalone_minimal_macho_x86_64,build,test
@swift-ci test with toolchain and preset

[ian-twilightcoder]

@swift-ci smoke test

[ian-twilightcoder]

@swift-ci test

[ian-twilightcoder]

Talked to Kuba a little bit, reflection was turned off "because it's a bit heavyweight". I'm not sure what the correct solution really is though. The OS/SDK owns the Darwin module, and the Darwin module requires reflection. Everything in stdlib/private (transitively) requires the Darwin module. So I don't see how it's possible to turn off reflection without doing something to migrate stdlib/private.

Currently stdlib/private is relying on all kinds of OS services like thread and process management, memory, etc., especially in the StdlibUnittest library which is used by ~300 tests. I don't know if the right thing is to turn all of those tests off in non-reflection mode, or if we need to make StdlibUnittest a host library rather than a platform library, or if we need some kind of PlatformMinimal overlay that doesn't try to use the host OS/SDK headers which don't match the target environment?

For now I'm trying to re-enable reflection to see if that clears up the immediate issue and then we can take more time to figure out what "minimal" means and how best to accomplish that.