Fix a thinko in the memory allocator that was causing us to strand an allocation range
in all but one of the AllocatedBlocks that matched the requested permissions.

Over time this would make the performance of expressions slow down considerably.

Also added a little bit of logging that was helpful in resolving the issue.

<rdar://problem/17954438>


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@215239 91177308-0d34-0410-b5e6-96231b3b80d8