Move the post-build step that creates lldb.py.

Being in lldb\source, ${CMAKE_CURRENT_BINARY_DIR} would resolve to
the build\tools\lldb\source directory.  For correct operation, and
parity with the shell script, it needs to resolve to the
build\tools\lldb\scripts directory.

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@212760 91177308-0d34-0410-b5e6-96231b3b80d8