Feature request: sizeof

[sivansh11]

problem described in #5872

possible syntax

[layout(std430)]
struct some_struct_t {
  ...
};
static_assert(sizeof(some_struct_t) == X, "sizeof(some_struct_t) != X");

or

struct some_struct_t {
  ...
};
static_assert(sizeof(some_struct_t, std430) == X, "sizeof(some_struct_t) != X");