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 enum1;

void main()
{
    enum {
        HELLO,
        WORLD
    }

    assert(HELLO == 0);
    assert(WORLD == 1);
}