Enable `-Wunused` #362

If merged, this would be used downstream in https://github.com/bitcoin/bitcoin/pull/36167.

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• #342 (Allow request cancellation for wrapped C++ methods by xyzconstant)

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Not quite sure what is happening here. The job is failing with the following (https://github.com/bitcoin-core/libmultiprocess/actions/runs/34366056349/job/102514949616?pr=362#step:6:253):

Warning: include-what-you-use reported diagnostics:
error: unknown warning option '-Wtrailing-whitespace'; did you mean '-Wunicode-whitespace'? [-Werror,-Wunknown-warning-option]

However -Wtrailing-whitespace already exists in the config on master, so I don't see why the CI would be passing on the master branch, but failing with that issue here. From what I can tell, even though GCC is the compiler being used by CMake (where this flag is supported), include-what-you-use is parsing using Clang, which doesn't support the flag.

Adding -Wno-unused-command-line-argument fixes the unknown warning option error (that makes sense as a side-effect, given the -Wunused addition), however the job is still failing with IWYU issues, that aren't present on master?