Auto merge of rust-lang#106371 - RalfJung:no-ret-position-noalias, r=nikic

do not add noalias in return position

`noalias` as a return attribute in LLVM indicates that the returned pointer does not alias anything else that is reachable from the caller, *including things reachable before this function call*. This is clearly not the case with a function like `fn id(Box<T>) -> Box<T>`, so we cannot use this attribute.

Fixes rust-lang/unsafe-code-guidelines#385 (including an actual miscompilation that `@comex` managed to produce).

Author: bors