[lldb] Replace radars link with documenting comment (NFC)

This replaces two radar links with improved comments explaining the
underlying issues.

 - The first issue is working around a compiler bug that was fixed in
   f454dfb6.
 - The second issue is an invariant that doesn't actually hold. The
   latter was marked as FIXME but there was nothing in the radar about a
   possible alternative solution.

Both radars were closed.