Implements `As<Blah>()` and `Is<Blah>()` automatically.
There are several benefits to using this over the pattern of hand-rolled `IsBlah()`, `AsBlah()` methods:
(1) We don't have to maintain a whole lot of hand written code.
(2) These allow us to cast from the base type to _any_ derived type in a single cast. The existing hand-rolled methods usually require a couple of intermediary casts to go from the base type to the leaf type.
(3) The use of a template parameter means these casts can be called from other template logic.
Note: Unlike the hand-rolled `AsBlah()` methods, it is safe to call `As<T>()` even if the type does not derive from `T`. If the object does not derive from `T` then `As` will simply return `nullptr`. This allows the calling logic to replace the common pattern of:
```
if (obj.IsBlah()) {
auto* b = obj.AsBlah();
...
}
```
with:
```
if (auto* b = obj.As<Blah>()) {
...
}
```
This halves the number of virtual method calls, and is one line shorter.
Change-Id: I4312e9831d7de6703a97184640864b8050a34177
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/34260
Reviewed-by: dan sinclair <dsinclair@chromium.org>
Commit-Queue: Ben Clayton <bclayton@google.com>
Ben Clayton