suport #[must_use] annotation for function calls whose return value must be read

[matthargett]

I had this bug today:

bit32.bor(fallbackColdFigment.flags, Emplacement)
return fallbackColdFigment.flags

which should have been

fallbackColdFigment.flags = bit32.bor(fallbackColdFigment.flags, Emplacement)
return fallbackColdFigment.flags

it would be nice if we could annotate the bit32 functions in the rule toml files to require the return value to be stored/read so it would have told me about the bug. Similarly, I would expect this potential bug to be found:

local fallbackColdFigment = bit32.bor(fallbackColdFigment.flags, Emplacement)
return fallbackColdFigments

where the return value is stored, but not read