The use of `operator<<` gets much more convoluted as things are changed
over to pointers and with inheritance. This CL switches the `operator<<`
methods to `ToString` functions.
Bug: tint:1718
Change-Id: I85fd25b870d82d995eb27014c767abe071e543b0
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/112046
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Dan Sinclair <dsinclair@chromium.org>
Reviewed-by: Ben Clayton <bclayton@google.com>