Added a stripModifiers() function to remove shared|const|immutable storage classes in D2 (should eventually be moved to a dhelpers file rather than llvm helpers). Replaced a few occurances of STCinvariant with STCimmutable.

void main()
{
    S t;
}

struct T
{
    int i;
    S* s;
}

struct S
{
    long i;
    T* t;
}