cmd/compile: heap allocation of go:notinheap type via string conversion

[mdempsky]

cmd/compile accepts this program, even though it involves a heap allocation of a go:notinheap type:

package p

//go:notinheap
type gcBits uint8

var x []gcBits

func f() {
        x = []gcBits("foo")    // ([]gcBits)("foo") escapes to heap
}

Unlikely that this would ever be written within package runtime, but seems worth preventing. I think it's just a matter of disallowing string -> []T conversions where T is go:notinheap, like how *T to *U conversions are already special-cased for go:notinheap in convertop.

/cc @aclements

[gopherbot]

Change https://golang.org/cl/142719 mentions this issue: cmd/compile: disallow converting string to notinheap slice