Add a function to make a mutex-protected copy of a breakpoint site's

Add a function to make a mutex-protected copy of a breakpoint site's
owners list, so the StopInfo machinery can get the list of owners without
some other thread being able to mess up the list by deleting/disabline one of its
locations in the process of doing so.

<rdar://problem/18685197>

llvm-svn: 243541