In particular macros that are only checked for definition or are
expanded during the evaluation of an #if or #elif directive are now also
added to the list available through Document::macroUses().
The names of undefined macros that are interesting (because they're used
in an #ifdef or a defined(...)) are now available through
Document::undefinedMacroUses().
Reviewed-by: Roberto Raggi <roberto.raggi@nokia.com>
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