cmd/compile: require -lang=go1.14 for overlapping interfaces

Support for overlapping interfaces is a new (proposed) Go language
feature to be supported in Go 1.14, so it shouldn't be supported under
-lang=go1.13 or earlier.

Fixes #34329.

Change-Id: I5fea5716b7d135476980bc40b4f6e8c611b67735
Reviewed-on: https://go-review.googlesource.com/c/go/+/195678
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

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

@@ -34,7 +34,7 @@ func expandiface(t *types.Type) {
 		switch prev := seen[m.Sym]; {
 		case prev == nil:
 			seen[m.Sym] = m
-		case !explicit && types.Identical(m.Type, prev.Type):
+		case langSupported(1, 14) && !explicit && types.Identical(m.Type, prev.Type):
 			return
 		default:
 			yyerrorl(m.Pos, "duplicate method %s", m.Sym.Name)