Union Initializer Fails During CIRGen

[bruteforceboy]

The following code snippet fails when generating CIR, and that didn't happen until PR#1007 was merged.

typedef struct {
  union {
    int a;
    long b;
  };
} S;

S s = { .a = 1 };

It seems there is some extra padding even after unpacking the union, and there is an error when computing the size?

I tried to fix it myself, but then it fails again when lowering to LLVM, because b is marked as inactive, and a is the "active" field, but when lowering to LLVM, the type converter uses the larger member of the union.

cc: @bcardosolopes and @ChuanqiXu9 since you authored the PR)