Instantiated generic struct layout is reused across different instantiating array sizes

[mlbrooks]

In the example, y is supposed to be three bytes long, with a .second field naming the third byte. But declaring x before y incorretly changes the actual size and layout of y, to be the same as x's.

Note that the types that instante mypair for x and y are different, but they belong to the same equivalence class under array-pointer decay.

Example:

forall( dtype T1, dtype T2 | sized(T1) | sized(T2) )
struct mypair {
    T1 first;
    T2 second;
};

int main() {

  #ifdef DECLARE_X
    mypair(char[1], char) x @= {};
    printf("|x| = %ld\n", sizeof(x));    // expect 2
  #endif

    mypair(char[2], char) y @= {};
    printf("|y| = %ld\n", sizeof(y));    // expect 3

    y.second = 0;
    printf("y.second = %d\n", y.second); // expect 0

    y.first[1] = 17;                     // in bound of char[2]
    printf("y.second = %d\n", y.second); // expect 0
}

Compiled plain, actual and expected:

|y| = 3
y.second = 0
y.second = 0

Compiled -DDECLARE_X, actual:

|x| = 2
|y| = 2
y.second = 0
y.second = 17

Compiled -DDECLARE_X, expected:

|x| = 2
|y| = 3
y.second = 0
y.second = 0

[mlbrooks]

Fixed by f9ad69d.