This Cl adds an ast::Builder class which provides a series of helper
methods to make creating AST nodes simpler.
Change-Id: Ife57f27e598d575681f7192d65fab968191699b1
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/32560
Reviewed-by: David Neto <dneto@google.com>
Commit-Queue: dan sinclair <dsinclair@chromium.org>
