This is the first commit to implementing an IR -> AST conversion path.
Not much currently implemented, but enough to start building upon.
Started implementing AST -> IR -> AST roundtrip tests.
Bug: tint:1902
Change-Id: I17fa9f3bc609b48ecb916ab47ae61bfa36821b44
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/131180
Reviewed-by: Dan Sinclair <dsinclair@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
