[llvm] [Offload] Make MemoryManager threshold ENV var size_t type. (PR #124063)

[Joseph Huber via llvm-commits]

jhuber6 wrote:

> > Can we use `uint64_t` instead?
> 
> First change I thought of was indeed the use of UInt64Envar, but the variable being assigned to is size_t and the actual use of the value in the MemoryManager constructor is also size_t. So, I thought (and still think) it is more prudent to use size_t. If size_t sized ENV var isn't a great idea, I'd like to learn why and will change my PR back to UInt64Envar.

We don't support any 32-bit platforms at all right now, and generally I don't like `size_t` because it varies by platform. It's just much easier to make it 64-bit and call it a day without worrying about how behavior changes with whatever build.

https://github.com/llvm/llvm-project/pull/124063