transform: API cleanup

Transforms are meant to be reusable. Those that hold state cannot be used concurrently.
State leakage between runs is dangerous.
To fix this:
* Add transform::Data - A new base class for extra information emitted by transforms.
* Add transform::DataMap - A container of Data, keyed by type.
* Add a transform::DataMap field to Transform::Output.
* Have FirstIndexOffset emit a FirstIndexOffset::Data.
* Deprecate the getters on the transform.

Mutability of the transform config is also dangerous as setters can be called while a transform is actively running on another thread.
To fix:
* Expose a VertexPulling::Config structure and add a constructor that accepts this.
* Deprecate the setters on VertexPulling.

Also deprecate Transform::Output::diagnostics.
Put all the transform diagnostics into the returned Program. Reduces error handling of the client.

Change-Id: Ibd228dc2fbf004ede4720e2d6019c024bc5934d1
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/42264
Reviewed-by: dan sinclair <dsinclair@chromium.org>
Reviewed-by: Ryan Harrison <rharrison@chromium.org>
Commit-Queue: Ben Clayton <bclayton@google.com>

fuzzers/tint_common_fuzzer.cc

@@ -144,7 +144,7 @@ int CommonFuzzer::Run(const uint8_t* data, size_t size) {
 
   if (transform_manager_) {
     auto out = transform_manager_->Run(&program);
-    if (out.diagnostics.contains_errors()) {
+    if (!out.program.IsValid()) {
       return 0;
     }