[R] [C++] Fix identifier ... preceded by whitespace warnings

[jonkeane]

Describe the enhancement requested

On r-devel-linux-x86_64-fedora-clang we are getting some compiler warnings. The first two are in cpp11, so I think we can ignore (there's already an upstream issue r-lib/cpp11#447). But the rest are in our code and we should probably address them before submitting.

checking whether package ‘arrow’ can be installed ... [91m/56m] WARNING
Found the following significant warnings:
  /data/gannet/ripley/R/test-clang/cpp11/include/cpp11/R.hpp:52:31: warning: identifier '_xl' preceded by whitespace in a literal operator declaration is deprecated [-Wdeprecated-literal-operator]
  /data/gannet/ripley/R/test-clang/cpp11/include/cpp11/named_arg.hpp:42:29: warning: identifier '_nm' preceded by whitespace in a literal operator declaration is deprecated [-Wdeprecated-literal-operator]
  /data/gannet/ripley/R/packages/tests-clang/arrow/libarrow/arrow-18.1.0/include/arrow/vendored/datetime/date.h:966:34: warning: identifier '_d' preceded by whitespace in a literal operator declaration is deprecated [-Wdeprecated-literal-operator]
  /data/gannet/ripley/R/packages/tests-clang/arrow/libarrow/arrow-18.1.0/include/arrow/vendored/datetime/date.h:967:34: warning: identifier '_y' preceded by whitespace in a literal operator declaration is deprecated [-Wdeprecated-literal-operator]
  /data/gannet/ripley/R/packages/tests-clang/arrow/libarrow/arrow-18.1.0/include/arrow/vendored/datetime/date.h:1975:13: warning: identifier '_d' preceded by whitespace in a literal operator declaration is deprecated [-Wdeprecated-literal-operator]
  /data/gannet/ripley/R/packages/tests-clang/arrow/libarrow/arrow-18.1.0/include/arrow/vendored/datetime/date.h:1983:13: warning: identifier '_y' preceded by whitespace in a literal operator declaration is deprecated [-Wdeprecated-literal-operator]
  filesystem.cpp:261:37: warning: identifier '_nm' preceded by whitespace in a literal operator declaration is deprecated [-Wdeprecated-literal-operator]
See the [install log](https://www.r-project.org/nosvn/R.check/r-devel-linux-x86_64-fedora-clang/arrow-00install.html) for details.

Component(s)

C++