The current implementation of `-application-extension` has a problem that affects the generation of ObjC headers for regular Swift modules.
The primary purpose of `-application-extension` is to prevent the use of unavailable APIs in app extensions. However, it has an impact on the generation of -Swift.h headers and exposes Swift's internal declarations to ObjC. This behavior is appropriate for mixed modules that are not consumed externally, such as app extensions, but it fails to address the situation when a module is not an extension itself but is consumed by the extension (c90cd11).
To resolve this issue while maintaining the desired behavior, we can introduce a new flag for this particular use-case.
This does several different things to improve how platforms are described in availability diagnostics:
• Mentions the platform in diagnostics for platform-specific @available(unavailable) attributes.
• Replaces “OS X” with “macOS”.
• Replaces “fooOS application extension” with “application extensions for fooOS”.
• Replaces “on fooOS” with “in fooOS”.
Fixes <rdar://problem/49963341>.
