AstGen: allow unary float builtins to forward a result location

[Rexicon226]

Release Notes

Previously Zig would not forward a result-location through the following builtin functions,

@sqrt
@sin
@cos
@tan
@exp
@exp2
@log
@log2
@log10
@floor
@ceil
@trunc
@round

This has now been changed. Where previous you couldn't write,

const x: f64 = @sqrt(@floatFromInt(N));

since @sqrt would not forward the f64 result type to @floatFromInt, now you can.

[alexrp]

Just a reminder that, if accepted, this should have some copy-pastable release notes in the PR description.

[Rexicon226]

Yeah, happy to write them up.

[mlugg]

One nitpick. If you fix that and rebase (I see at least one writergate-related change you'll need to make), I'll be happy to merge (ping me if I forget).