[flang] Fix error compiling std::min on macos

On macos, `size_t` is `unsigned long` while `size_t - int64_t` is
`unsigned long long` so std::min requires an explicit type to compile.

Differential Revision: https://reviews.llvm.org/D99340

flang/unittests/Runtime/buffer.cpp

@@ -31,7 +31,7 @@ public:
           static_cast<int>(at), static_cast<int>(minBytes),
           static_cast<int>(maxBytes));
     }
-    auto result{std::min(maxBytes, bytes_ - at)};
+    auto result{std::min<std::size_t>(maxBytes, bytes_ - at)};
     std::memcpy(to, &data_[at], result);
     expect_ = at + result;
     return result;