Generate and validate all entry points individually.
This is required since GLSL has separate shader files, and
can only have a single "main" entry point.
Bug: tint:1217
Change-Id: Ie5cb510aaef3b7c8a7573f5fa9446815284afecb
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/61920
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Stephen White <senorblanco@chromium.org>
Reviewed-by: Ben Clayton <bclayton@google.com>
Stephen White