cmd/compile: improve error message for unknown fields in structs

Improves the error message by moving the field name before the body
of a struct, in the error message for unknown fields for structs.

* Exhibit:
Given program:

package main

import "time"

func main() {
  _ = struct {
    about      string
    before     map[string]uint
    update     map[string]int
    updateTime time.Time
    expect     map[string]int
  }{
    about:   "this one",
    updates: map[string]int{"gopher": 10},
  }
}

* Before:
./issue17631.go:20: unknown struct { about string; before map[string]uint;
 update map[string]int; updateTime time.Time; expect map[string]int } field
'updates' in struct literal

* After:
./issue17631.go:20: unknown field 'updates' in struct literal of type { about string;
before map[string]uint; update map[string]int; updateTime time.Time;
expect map[string]int }

Fixes #17631

Change-Id: I76842616411b931b5ad7a76bd42860dfde7739f4
Reviewed-on: https://go-review.googlesource.com/32240
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

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

@@ -3095,7 +3095,7 @@ func typecheckcomplit(n *Node) *Node {
 
 				f := lookdot1(nil, l.Sym, t, t.Fields(), 0)
 				if f == nil {
-					yyerror("unknown %v field '%v' in struct literal", t, l.Sym)
+					yyerror("unknown field '%v' in struct literal of type %v", l.Sym, t)
 					continue
 				}
 				fielddup(f.Sym.Name, hash)

test/fixedbugs/issue14136.go

@@ -14,6 +14,6 @@ package main
 type T struct{}
 
 func main() {
-	t := T{X: 1, X: 1, X: 1, X: 1, X: 1, X: 1, X: 1, X: 1, X: 1, X: 1} // ERROR "unknown T field"
+	t := T{X: 1, X: 1, X: 1, X: 1, X: 1, X: 1, X: 1, X: 1, X: 1, X: 1} // ERROR "unknown field 'X' in struct literal of type T"
 	var s string = 1 // ERROR "cannot use 1"
 }

test/fixedbugs/issue17631.go

@@ -0,0 +1,22 @@
+// errorcheck
+
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import "time"
+
+func main() {
+	_ = struct {
+		about      string
+		before     map[string]uint
+		update     map[string]int
+		updateTime time.Time
+		expect     map[string]int
+	}{
+		about:   "this one",
+		updates: map[string]int{"gopher": 10}, // ERROR "unknown field 'updates' in struct literal of type"
+	}
+}