Fix iOSDarwin()'s doc comment.

Summary:
I saw the doc comment using the Option + Click in Xcode and I was
confused by the fact that tvOS wasn't listed. The method definition
also doesn't make it clear that the check for tvOS is taking place as
the tvOS check is hidden in the isiOS() check.

Reviewers: rjmccall

Reviewed By: rjmccall

Subscribers: dexonsmith, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70364

This patch is from Varun Gandhi <varun_gandhi@apple.com>