add noalias attribute to ~ return values

[thestinger]

This also removes a FIXME I added referring to a now closed issue.

[huonw]

Is this valid for ~@mut T, or do these chains not count as aliasing for LLVM?

[sanxiyn]

@huonw As I understand it does not count as aliasing for LLVM.

[huonw]

Even:

fn f(x: @mut int) -> (~@mut int, ~@mut int) { (~x, ~x) }

?

[sanxiyn]

@huonw In that case, f's return value is not ~ pointer at all.

[thestinger]

The noalias attribute is does not apply recursively. It's only relevant for the outermost pointer. It's not a generic alias analysis tool as it only works on the outermost pointer on parameters and return values so it's just a nice to have stopgap before TBAA.