Amend SE-0224 to include #if compiler as well.

[tkremenek]

This is based on feedback in the review.

[tkremenek]

@danielmartin This amends SE-0224 to include if compiler, which your implementation supports. The plan is to accept SE-0224 with this revision (which is already reflected in the implementation). Does this look good to you?

[benrimmington]

@tkremenek Will the #if !compiler(>=4.2) and #if compiler(<4.2) conditions always be false?

I've just noticed that the #if compiler tests (added by @hartbit for SE-0212) were accidentally removed in swiftlang/swift#17689

[danielmartin]

@tkremenek Looks good to me! I added some basic #if compiler tests to my implementation to prevent this feature to ever regress.

[tkremenek]

@danielmartin That's great. Thanks!