[dev.regabi] cmd/compile: remove CTRUNE

Since CL 255217, we've been able to rely on types.UntypedRune to
identify untyped rune literals, rather than needing Mpint.Rune /
CTRUNE. This makes way for switching to using go/constant, which
doesn't have a separate notion of rune constants distinct from integer
constants.

Passes toolstash-check.

Change-Id: I319861f4758aeea17345c101b167cb307e706a0e
Reviewed-on: https://go-review.googlesource.com/c/go/+/272652
Reviewed-by: Robert Griesemer <gri@golang.org>
Trust: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/gc/noder.go

@@ -656,6 +656,9 @@ func (p *noder) expr(expr syntax.Expr) *Node {
 		return p.mkname(expr)
 	case *syntax.BasicLit:
 		n := nodlit(p.basicLit(expr))
+		if expr.Kind == syntax.RuneLit {
+			n.Type = types.UntypedRune
+		}
 		n.SetDiag(expr.Bad) // avoid follow-on errors if there was a syntax error
 		return n
 	case *syntax.CompositeLit:
@@ -1428,7 +1431,6 @@ func (p *noder) basicLit(lit *syntax.BasicLit) Val {
 
 	case syntax.RuneLit:
 		x := new(Mpint)
-		x.Rune = true
 		if !lit.Bad {
 			u, _ := strconv.Unquote(s)
 			var r rune