The Presburger library currently uses int64_t throughout for its integers.
This runs the risk of silently producing incorrect results when overflows occur.
Fixing this issue requires some sort of multiprecision integer
that transparently supports aribtrary arithmetic computations.
The class SlowMPInt provides this functionality, and is intended to be used
as the slow path fallback for a more optimized upcoming class, MPInt, that optimizes
for the Presburger library's workloads.
Reviewed By: ftynse
Differential Revision: https://reviews.llvm.org/D123758
