tweak promoted fields diag more

Before:
tst.go:13:5: error: name 'int' is ambiguous, could be any of 't2.t', 'u2.u'
  v.int
    ^

Now:
tst.go:13:5: error: name 'int' is ambiguous
  v.int
    ^
tst.go:3:17: note: could be in 't2.t'
type t struct { int }
                ^
tst.go:4:17: note: could be in 'u2.u'
type u struct { int }
                ^