[TypeChecker] Rephrase platforms in availability diagnostics
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>.
