Lock the access to the BreakpointLocationCollection.

I was investigating an odd crash in lldb when the breakpoint site
goes to bump the hit counts of the locations it implements.  I noticed
that the BreakpointLocationCollection wasn't locking itself for access and
modification.  I don't see how that can cause the crash I'm seeing, but still
this is the right thing to do...

<rdar://problem/25178205>

llvm-svn: 270939