Revert "Remove rdar links; NFC"

This reverts commit d618f1c3.
This commit wasn't reviewed ahead of time and significant concerns were
raised immediately after it landed. According to our developer policy
this warrants immediate revert of the commit.

https://llvm.org/docs/DeveloperPolicy.html#patch-reversion-policy

Differential Revision: https://reviews.llvm.org/D155509