We did warn on them in some cases, but not others. In particular, if one
used a slice composite literal with struct pointer elements, and omitted
the type of an element's composite literal, it would not get any warning
even if it should get one.

The issue is that typ.Underlying() can be of type *types.Pointer. Skip
those levels of indirection before checking for a *types.Struct
underlying type.

isLocalType also needed a bit of tweaking to ignore dereferences.
Perhaps that can be rewritten now that we have type info, but let's
leave it for another time.

Fixes #23539.

Change-Id: I727a497284df1325b70d47a756519f5db1add25d
Reviewed-on: https://go-review.googlesource.com/89715
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>