Resolve overload ambiguity on Mac OS when printing size_t in diagnostics

Precommit builds cover Linux and Windows, but this ambiguity would only
show up on Mac OS: there we have int32_t = int, int64_t = long long and
size_t = unsigned long. So printing a size_t, while successful on the
other two architectures, cannot be unambiguously resolved on Mac OS.

This is not really meant to support printing arguments of type long or
size_t, but more as a way to prevent build breakage that would not be
detected in precommit builds, as happened in D125429.

Technically we have no guarantee that one of these types has the 64 bits
that afdac5fb wanted to provide, so proposals are welcome. We do
have a guarantee though that these three types are different, so we
should be fine with overload resolution.

Reviewed By: aeubanks

Differential Revision: https://reviews.llvm.org/D125580