[LLDB] Replacing use of ul suffix in GetMaxU64Bitfield since it not guarenteed to be 64 bit

GetMaxU64Bitfield(...) uses the ul suffix but we require a 64 bit unsigned integer and ul could be 32 bit. So this replacing it with a explicit cast and refactors the code around it to use an early exit.

Differential Revision: https://reviews.llvm.org/D70992