Enforce all places where Dawn passes in or returns a ast::Module, now takes a `const Program* ` or returns a `Program`.
As the end goal of all this is to have immutable Programs, all Program inputs take a pointer instead of moving the actual object.
As consumers of a Program are now all const, we have to const_cast to work around all the places we've been incorrectly mutating a ast::Module.
These const_casts are temporary, and will be fixed in the next set of changes.
Depends on https://dawn-review.googlesource.com/c/dawn/+/38522
Bug: tint:390
Change-Id: Ie05b112b16134937d1b601e9b713ea4ec4e1c677
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/38541
Reviewed-by: dan sinclair <dsinclair@chromium.org>
