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 bug52;
struct Vec { double x,y,z; }
struct Pair(T, U) { T first; U second; }
typedef Pair!(double, Vec) Hit;
void main() {}