C++: Remove FPs from cpp/too-few-arguments

[calumgrant]

Fixes https://github.com/github/codeql-c-team/issues/2462

False-positives in this query were due to implicit C function definitions. Arguably, we shouldn't report this alert for implicit C function definitions, because these are much more likely to be extraction errors rather than actual problems.

The current tests pretty well covered the existing behaviour, so I just reused this test.

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