cpp: allow casting C++ struct reference types to C types

This is safe because the C++ layout is static_assert'ed to be the
same as the C layout. It removes bad-looking reinterpret_cast
from application code.

Mutable reference casts are not allowed. This is because:
 - Some C++ structs have RAII objects in them. Casting to C and
   then changing the members could cause unexpected lifetime
   changes
 - Structs with "free members" functions only get const conversion
   operators. This is because output structs like this have RAII data
   which should not be mutated. The C structs do not have
   const-qualified members, so conversion to a mutable C struct would
   be unsafe.

Bug: chromium:40195122
Change-Id: I18cd06b04da2ea34803f517e2b640ceaaba408af
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/185966
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Commit-Queue: Austin Eng <enga@chromium.org>
Reviewed-by: Loko Kung <lokokung@google.com>