Remove noalias on return values

[dotdash]

On return values noalias has a stricter meaning than on arguments. It
means that the function is malloc-like. To quote the LLVM docs:

Furthermore, the semantics of the noalias attribute on return values
are stronger than the semantics of the attribute when used on
function arguments. On function return values, the noalias attribute
indicates that the function acts like a system memory allocation
function, returning a pointer to allocated storage disjoint from the
storage for any other object accessible to the caller.

We use it for all kinds of functions, which is wrong, so let's stop
doing that.

Fixes #21996

[rust-highfive]

r? @nick29581

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[alexcrichton]

@bors: r+ 7fbcdf6 rollup

[alexcrichton]

@bors: r-

[dotdash]

I just realized that the rationale given here makes no sense at all. The attribute is on a sret argument, not actually a return value. Will need more analysis

[dotdash]

Yeah, this just worked around the actual issue that caused #21996.