c: relax constexpr constraints, fix alignof

- Allow short-circuiting of constant logical expressions where the
  unevaluated operand is not a constant expression (`1 || 0/0`)

- Allow constant integer expressions that evaluate to zero to be used
  as null pointer constants (GNU extension).

    + According to the standard, `int *x = 5*0;` should be rejected.
      But compilers evaluate `5*0 -> 0` and allow it as if a null pointer
      literal.

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