[spirv-writer] Don't hash `const char*`

Import into google3 is complaining that:
hash<const char*> would hash the pointer, not the contents of the
string, which is almost certainly not what you want.

A better follow-on solution may be pass an enum instead.

Change-Id: I34a4796e14e8cc64e5747e766f8b025eb3e2aefa
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/154641
Commit-Queue: Austin Eng <enga@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: James Price <jrprice@google.com>