[D-runtime] Bug 3150: requires changes to rt/lifetime.d

[Don Clugston]

Fixing this bug will be important for 64 bits:
3150 cast from dynamic array to ulong is allowed

As discussed in the comments to that bug, a change is required at two
places in druntime, because druntime currently relies on the compiler
bug. I don't think it's particularly difficult, but I don't want to do
it myself. Can one of you guys take care of it?