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

[via llvm-commits]

hidekisaito 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.

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