Add helper functions to iterate over ChainedStructs

This CL adds two helpers for more ergonomic processing of
ChainedStructs.

1. FindInChain(): Iterates through the chain and automatically
   casts the ChainedStruct into the appropriate child type before
   returning.
2. ValidateSTypes(): Verifies that the chain only contains structs
   with sTypes from a pre-defined set. This also allows the caller
   to specify one-of constraints.
3. ValidateSingleSType(): Verifies that the chain contains a
   single struct with a specific sType or is an empty chain. This
   is a common case of |ValidateSTypes()| and is separated out as
   a fast-path.

Change-Id: I938df0bf2a9b1800b1105fb7f80fbde20bef8ec8
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/47680
Commit-Queue: Brian Ho <hob@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>