[lldb][lit] Guard MallocNanoZone envvar in shell tests (#88824)

Previously the MallocNanoZone envvar would be set to 0 on Darwin for the
LLDB shell tests, but this should guarded behind ASan being enabled as
opposed to simply running the test suite behind Darwin. This required
that the LLVM_USE_SANITIZER option be added as an attribute to the lit
config for shell tests.