Change behavior with zero-sized static array extents

Currently, Clang previously diagnosed this code by default:
  void f(int a[static 0]);
saying that "static has no effect on zero-length arrays", which was
accurate.

However, static array extents require that the caller of the function
pass a nonnull pointer to an array of *at least* that number of
elements, but it can pass more (see C17 6.7.6.3p6). Given that we allow
zero-sized arrays as a GNU extension and that it's valid to pass more
elements than specified by the static array extent, we now support
zero-sized static array extents with the usual semantics because it can
be useful in cases like:

  void my_bzero(char p[static 0], int n);
  my_bzero(&c+1, 0); //ok
  my_bzero(t+k,n-k); //ok, pattern from actual code