Made the Materializer not write back variables
if they didn't change, just like it does for
registers.  This makes life easier for kernel
debugging and any other situation where values
are read-only.

<rdar://problem/16367795>


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@204764 91177308-0d34-0410-b5e6-96231b3b80d8