hclsyntax: Highlight key expression on invalid key type in object cons

Previously we were incorrectly indicating the value expression as the
problem.

This fixes #54.

hcl/hclsyntax/expression.go

@@ -727,8 +727,8 @@ func (e *ObjectConsExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnostics
 				Severity:    hcl.DiagError,
 				Summary:     "Incorrect key type",
 				Detail:      fmt.Sprintf("Can't use this value as a key: %s.", err.Error()),
-				Subject:     item.ValueExpr.Range().Ptr(),
-				Expression:  item.ValueExpr,
+				Subject:     item.KeyExpr.Range().Ptr(),
+				Expression:  item.KeyExpr,
 				EvalContext: ctx,
 			})
 			known = false

hcl/hclsyntax/expression_test.go

@@ -403,6 +403,12 @@ upper(
 			}),
 			0,
 		},
+		{
+			`{[]: "yes"}`,
+			nil,
+			cty.DynamicVal,
+			1, // Incorrect key type; Can't use this value as a key: string required
+		},
 		{
 			`{"hello" = "world", "goodbye" = "cruel world"}`,
 			nil,