Improve the error message for wrong
case-field names in composite literals,
by mentioning the correct field name.

Given the program:
package main

type it struct {
        ID string
}

func main() {
        i1 := &it{id: "Bar"}
}

just like we do for usage of fields, we now
report wrongly cased fields as hints to give:

ts.go:8:14: unknown field 'id' in struct literal of type it (but does have ID)

instead of before:

ts.go:8:14: unknown field 'id' in struct literal of type it

Fixes #22794

Change-Id: I18cd70e75817025cb1df083503cae306e8d659fd
Reviewed-on: https://go-review.googlesource.com/78545
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>