The WGSL spec requires initializers for Output, Private and Function
variables. This CL adds initializers where needed.
Bug: tint:75
Change-Id: Id97f85a67ead2ffc41d6bdd1b71bf7034b04502a
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/20980
Reviewed-by: David Neto <dneto@google.com>