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

[tskeith]

Summary

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.

[klausler]

Thank you.

flang/unittests/Runtime/buffer.cpp

if (enforceSequence_ && at != expect_) {
       handler.SignalError("Read(%d,%d,%d) not at expected %d",
           static_cast<int>(at), static_cast<int>(minBytes),
           static_cast<int>(maxBytes), static_cast<int>(expect_));
     } else if (at < 0 || at + minBytes > bytes_) {
       handler.SignalError("Read(%d,%d,%d) is out of bounds",
           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;
   }
   std::size_t Write(FileOffset at, const char *from, std::size_t bytes,
       IoErrorHandler &handler) {
     if (enforceSequence_ && at != expect_) {
       handler.SignalError("Write(%d,%d) not at expected %d",