[C2y] Claim conformance to WG14 N3363 (#130980)

This paper clarifies two things:
  * a call to va_start must be within the body of a variadic function,
  * the va_list arguments must be lvalues

Clang has correctly diagnosed the first point since Clang 5.0. Clang
generally diagnoses the second point, except for va_copy. However, the
second point is not a constraint violation, so a diagnostic is not
strictly required. That said, it would be good for us to properly
diagnose va_copy.

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