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 bug42;

void main() {
    int i = 2;
    switch (i) {
    case 0:
    case 1:
    default:
    }
}