For the most part, the differences between the diagnostics introduced
by the C++ implementation and the new SwiftIfConfig implementation are
cosmetic, so these are only wording changes.
The one major difference is that we've dropped the warnings about
potential typos in os/arch checks. For example, if one writes:
#if os(bisionos)
// ...
#endif
The C++ implementation will produce a warning "unknown operating system
for build configuration 'os'" with a note asking "did you mean
'visionOS'"? These warnings rely on a static list of known operating
systems and architectures, which is somewhat unfortunate: the whole
point of these checks is that the Swift you're dealing with might not
have support for those operating systems/architectures, so while these
warnings can be helpful in a few cases, they also cause false
positives when porting. Therefore, I chose not to bring them forward.
canImport should be able to take an additional parameter labeled by either version or
underlyingVersion. We need underlyingVersion for clang modules with Swift overlays because they
have separate version numbers. The library users are usually interested in checking the importability
of the underlying clang module instead of its Swift overlay.
Part of rdar://73992299
