Add a blackHole function

[Karl]

@xwu

In general these feel appropriate for XCTest perhaps (or, as you say, swift-benchmark). Not sure how I feel about having these in the standard library itself given their specific relevance for testing when Swift ships with a core library for testing.

I'm not sure how the existence of XCTest is relevant - surely we also support alternative benchmarking libraries? And those libraries should not be forced to depend on XCTest for fundamental functionality, IMO, nor should they have to second-guess the compiler. This feels like it should be some kind of compiler built-in; I think it should be exposed directly.

I wouldn't mind if this was part of some kind of supplementary library rather than the standard library itself, but it should have nothing to do with XCTest.

There's also the problem that, like black holes themselves, nobody knows what the evolution process for XCTest is.