Implement a !subst operation simmilar to $(subst) in GNU make to do
def/var/string substitution on generic pattern templates.  For example:

def Type;
def v4f32 : Type;
def TYPE : Type;

class GenType<Type t> {
  let type = !(subst TYPE, v4f32, t);
}

def TheType : GenType<TYPE>;

llvm-svn: 71801