[lldb] Get lldb-server platform's --socket-file working again

`lldb-server platform --socket-file /any/path` currently always fails to create
the socket file.  This stopped working after D67424 which changed the
input variables of `writeFileAtomically` slightly. We're expected to
pass in a temporary path template (`/tmp/foo-%%%%%`) and the final
path we want to write. Instead we currently pass in the never set
`temp_file_path` as the temporary path (which will make this function always
fail) and pass in the temp_file_spec's path as the final path (which is actually
the template path such as `/tmp/foo-%%%%%`) instead of the actual path
we want to write (e.g. `/tmp/foo`).

Reviewed By: labath

Differential Revision: https://reviews.llvm.org/D85890