Compiler swallows `blackHole(_:)`

[Karl]

Add a blackHole function

Hi! I've recently been using the excellent swift-benchmark package: GitHub - google/swift-benchmark: A swift library to benchmark code snippets. (which I hope Apple/Swift.org will adopt one day - it needs to be fleshed-out a bit, but it's really much better than XCTest, especially when it comes to linux support). One thing that I've found a bit awkward is that it doesn't come with a "black hole" function to prevent the compiler optimising values away. This kind of function appears to be used e…

I still think it is valuable for the compiler to provide built-in non-optimisable opaque functions for producing/consuming a value.