[CMake] Temporarily workaround CMake dependencies bug

[bnbarham]

Workaround a CMake bug in order to make sure that ASTGen is rebuilt when the swift-syntax dependencies change.

[bnbarham]

https://github.com/apple/swift-syntax/pull/1081/files

@swift-ci please test

[drodriguez]

I think this workaround has a little problem, but I am not completely sure.

Because the files are empty, it seems that the Swift compilation does not touch the .o files that results from them. This means that when SwiftSyntax is changed, the files will be touched, which will dirty the target, but because Swift does not update the mtime of the .o files the target will always be out-of-date and will be rebuilt all the time. Since this library is part of the compiler, this produces a cascading recompilation of almost all the compiler and the standard library. This is very disrupting when just modifying some test files, since it will rebuilt a large number of files, killing iteration time.

One solution is adding another command that removes the .o files (maybe others), but since the file structure in the build directory are not "public API", I am not sure about this solution being correct.

[bnbarham]

Because the files are empty, it seems that the Swift compilation does not touch the .o files that results from them.

This is a problem in general, ie. if there's a change that doesn't impact the .o file, it will continually be rebuilt. So even without this change, that was already the case for swift-syntax. It's "just" that now this cascades out further.

One solution is adding another command that removes the .o files (maybe others), but since the file structure in the build directory are not "public API", I am not sure about this solution being correct.

I think I never looked into doing this because just removing the .o files isn't enough without the workaround - the library would be different, but dependents still wouldn't be rebuilt as from CMake's perspective there isn't any reason to.

But, seems like we could do both. I'll try remove the .o's in the prebuild and see how that goes.