[dawn][native] *Don't* spanify ShaderSourceSPIRV

ShaderSourceSPIRV::codeSize is a uint32_t and not a size_t so it cannot
be made to transparently use a dawn::Span. Instead add helper function
that creates the Span<const uin32_t> and use it where applicable.

Also spanify SpirvValidation.h/.cpp as a side effect.

Adds an assert that the "length" that we spanify automatically in the
input API is a size_t.

Bug: 515272358
Change-Id: If2df29f57919db4e47f97a547bde782034a81442
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/320304
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Loko Kung <lokokung@google.com>