cmd/compile: duplicate index error should distinguish arrays and slices

[josharian]

package main

var s = []string{
	1: "dup",
	1: "dup",
}

https://play.golang.org/p/cncTK4uwake

Compile. Result:

./prog.go:5:2: duplicate index in array literal: 1

But the duplicate index is in a slice literal, not an array literal.

Probably an artifact of how cmd/compile used to use TARRAY to represent both arrays and slices.

Low priority, but probably also an easy fix. Might be a good first issue for someone interested in the compilerr.

[gopherbot]

Change https://golang.org/cl/204617 mentions this issue: cmd/compile: make duplicate index error distinguish arrays and slices