On macOS, we can't do the DYLD_INSERT_LIBRARIES trick with a shim
python binary as the ASan interceptors get loaded too late. Find the
"real" python binary, copy it, and invoke it.
Hopefully this makes the GreenDragon and swift-ci sanitizer bots
I tested this out by running `../llvm-macosx-x86_64/bin/llvm-lit test
--filter TestNSDictionarySynthetic.py` in an ASanified swift-lldb build
directory and it worked (i.e. no more "interceptors loaded too late"