Issue Description:

anonymous `async let` tasks, such as

async let _:Void = foo()

currently fail to compile, even though SE-0317 says this should be legal:

> Special attention needs to be given to the async let _ = ... form of declarations. This form is interesting because it creates a child-task of the right-hand-side initializer, however it actively chooses to ignore the result. Such a declaration (and the associated child-task) will run and be awaited-on implicitly, as the scope it was declared in is about to exit — the same way as an unused async let declaration would be.