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.
When we encounter a check like `#if compiler(>=6.0) && something` or
`#if swift(<6.0) || something`, and the left-hand term is a versioning
check that determines the result of the whole condition, then we will
not attempt to validate the right-hand term. This allows us to use
versioned checks along with new discovery features (say, if we add an
`#if attribute(x)`) without having to next conditions.
