go/types: collect type info for type ...T in variadic functions

Because the code type-checks T rather than ...T (and then corrects
the type to []T "manually"), it didn't automatically record the
type for the ast.Expr corresponding to ...T. Do it manually.

Fixes #28277.

Change-Id: I3d9aae310c90b01f52d189e70c48dd9007f72207
Reviewed-on: https://go-review.googlesource.com/c/143317


Reviewed-by: Alan Donovan <adonovan@google.com>