[5.1] [TypeChecker] Rephrase platforms in availability diagnostics

[beccadax]

This PR cherry-picks #24334 to swift-5.1-branch. Master PR reviewed by @jrose-apple.

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.

(If you want to review this, turn on "Hide whitespace changes" in the diff settings—this PR reindents a lot more code than it actually rewrites.)

[beccadax]

@swift-ci test