Change emitted name for `StageAttribute`.

There is no longer a specific `@stage` attribute in WGSL. Instead each
of the `@compute`, `@vertex` or `@fragment` attributes are used.
Internally they collapse into a `StageAttribute` object. Instead of
emitting `stage` as the name of the attribute, emit the specific name
for the pipeline stage used. This closer matches what was in the WGSL
file.

Bug: tint:1832
Change-Id: I6081f49c89fcfc9ee33bbb46ec85cedddeaf027d
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/180120
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: dan sinclair <dsinclair@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>