[Parse] Fix nested ifConfig compiler checks (#74415)
The parser is supposed to avoid looking inside unmatched `#if` compiler (et al) blocks.
This usually means that the following code builds fine
#if compiler(>=100)
foo bar
#endif
however, a logical bug meant that if the check was nested inside an already-inactive
`#if` block, it would not adhere to this evaluation-skipping behavior
#if false
#if compiler(>=100)
foo bar // error!
#endif
#endif
This PR fixes this specific case.
