[!7] Improve handling of user defined literal warning in clang

Merge branch 'feature/improve-handling-of-literal-warning-in-clang' into 'master'

ref:staging/dune-logging CMake does not correctly detect whether the compiler
supports disabling a given warning, as no diagnostic is emitted by default for
an unknown disabled warning flag.

So disable the warning directly in the header when clang is detected.

See merge request [!7]

  [!7]: gitlab.dune-project.org/staging/dune-logging/merge_requests/7
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