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.

module aa5;

void main()
{
    int[int] aa;
    aa[42] = 1;
    int i = aa[42];
    assert(i == 1);
}