This CL adds CallStatement to the AST to allow wrapping a CallExpression
into a statement.
Bug: tint:45
Change-Id: I5a9525514344fcc2cf9900eb2cd78c1fb94ca2ef
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/25320
Reviewed-by: David Neto <dneto@google.com>