[clang] Turn `-Wdeprecated-literal-operator` on by default (PR #111027)

[James Y Knight via cfe-commits]

jyknight wrote:

Some projects still seem to have a requirement for compatibility with GCC 4.8 -- which supports a lot of C++11 but didn't apply the change in CWG 1473, and thus _require_ the space in `operator"" _udl`. That is somewhat unfortunate.

Two examples from the first page of github search for "Wdeprecated-literal-operator":
- Adding back the deprecated space: https://github.com/mongodb/mongo-cxx-driver/pull/1194, effectively reverting the previous removal of the space in https://github.com/mongodb/mongo-cxx-driver/pull/1178.
- Trying to remove the space to fix the warning, but ending up using an ifdef due to GCC 4.8 compatibility: https://github.com/apache/arrow-nanoarrow/pull/661

https://github.com/llvm/llvm-project/pull/111027