[swiftSyntax] Fix issue where swiftc was not found when swiftSyntax is build via Xcode

[ahoppen]

swiftSyntax finds swiftc based on the directory layout of the build folder. However, that layout is different if swiftSyntax is built via Xcode than if it's built with Ninja.

For Ninja:

- bin/
  - swiftc
- lib/
  - swift/
    - ${target}/
      - libswiftSwiftSyntax.[dylib|so]

For Xcode:

- bin/
  - swiftc
- lib/
  - swift/
    - macosx/
      - x86_64/
        - libswiftSwiftSyntax.[dylib|so]

Allow swiftSyntax to find swiftc by traversing up to the lib folder and then moving to bin/ instead of hardcoding the nesting level of libswiftSwiftSyntax.dylib inside lib.

[ahoppen]

@swift-ci Please smoke test

[harlanhaskins]

Nitpick: unfinished comment

[harlanhaskins]

This is definitely an improvement, thanks!

[ahoppen]

@swift-ci Please smoke test and merge

[compnerd]

Hmm, is there a reason to not try to make the layout uniform across the two builds? It seems that it only complicates things to have the different layouts. Making the code more robust anyways is probably a good idea.

[harlanhaskins]

@compnerd I think this is the correct answer -- this library is going to be somewhere under lib/ on all platforms, so walking up to lib/ and then to bin/ seems like a good way to support all layouts.

I agree that the layouts should match across platforms, but that's a more invasive change.

[ahoppen]

@swift-ci Please smoke test and merge

[compnerd]

@harlanhaskins - also would be interesting to consider windows :-)

[compnerd]

Sorry @ahoppen about causing the conflicts. Should be relatively easy to get these changes moved on top of the changes that I had.

[harlanhaskins]

@compnerd Ah, yes! Apologies -- I am admittedly entirely uninformed of Windows library structure. What does the layout currently look like on Windows?

[ahoppen]

@swift-ci Please smoke test

[jrose-apple]

Yeah, this is incorrect, sorry. For Apple platforms, both Ninja and Xcode build into the architecture-specific directories first, but then lipo together all the architectures into the platform directory. If Xcode's not doing that for you we either have a CMake bug or you're building the wrong target.

[ahoppen]

@swift-ci Please smoke test

[ahoppen]

@compnerd This change still covers your use case, right?