This CL renames VariableStatement to VariableDeclStatement to make it
clearer what it was modeling.
Bug: tint:25
Change-Id: Idd0d27fad7cc402f286e1abad74092c9d1961d91
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/18341
Reviewed-by: David Neto <dneto@google.com>