[Parse] Fix nested ifConfig compiler checks

[kabiroberai]

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.

[kabiroberai]

I'm unsure if this fix needs to be ported to SwiftSyntax as well — I couldn't find the "skip parsing unmatched compiler checks" logic there. If it exists, could someone please point me to it?

[rintaro]

The change looks good to me. Thanks!
swift-syntax doesn't support skipping regions, for now. So no need to fix this in swift-syntax side.

@kabiroberai Could you squash the change and add relevant commit message? Or I if you write it here, I can squash and add the comment when merging.

[kabiroberai]

@rintaro feel free to use the issue title: "[Parse] Fix nested ifConfig compiler checks". Thanks!

[rintaro]

@swift-ci Please smoke test

[rintaro]

OK, but I hope to put more explanation in the comment.
Let's just use the description:

[Parse] Fix nested ifConfig compiler checks

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.