cmd/compile: better error message for failed string conversion

[griesemer]

https://play.golang.org/p/8ADwHokqcK5 complains with:

prog.go:6:12: cannot use x (type []T) as type []byte in argument to runtime.slicebytetostring

The error message should not mention runtime.slicebytetostring which is an implementation detail.

(There are probably more error situations that expose runtime calls this way. Should handle them all uniformly.)

[griesemer]

Has been like this for a while. Moving to 1.12.

[griesemer]

We should wait with fixing this depending on the resolution of #23536. It's not just go/types. The compiler also seems not consistent.