[asan] Do not unset DYLD_ROOT_PATH before calling atos on Darwin

We were unsetting DYLD_ROOT_PATH before calling atos on Darwin in order to
address it not working for symbolicating 32 bit binaries. (atos essentiall
tries to respawn as a 32 bit binary and it's disallowed to respawn if
DYLD_ROOT_PATH is set ... ) However, processes rely on having DYLD_ROOT_PATH
set under certain conditions, so this is not the right fix. In particular, this
always crashes when running ASanified process under the debugger in Xcode with
iOS simulator, which is a very important workflow for us to support.

This patch reverts the unsetting of the DYLD_ROOT_PATH. The correct fix to the
misbehavior on 32-bit binaries should happen inside atos.

http://reviews.llvm.org/D10722

llvm-svn: 240724