When the compiler decomposes a user variable, track its origin so that
it can be recomposed during DWARF generation.

Change-Id: Ia71c7f8e7f4d65f0652f1c97b0dda5d9cad41936
Reviewed-on: https://go-review.googlesource.com/50878Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>