Delete DataBufferMemoryMap.

After a series of patches on the LLVM side to get the mmaping
code up to compatibility with LLDB's needs, it is now ready
to go, which means LLDB's custom mmapping code is redundant.
So this patch deletes it all and uses LLVM's code instead.

In the future, we could take this one step further and delete
even the lldb DataBuffer base class and rely entirely on
LLVM's facilities, but this is a job for another day.

Differential Revision: https://reviews.llvm.org/D30054

llvm-svn: 296159