First the leak sanitizer has to be disabled, as even an empty script
leads to leak detection with Python.
Then we need to preload the ASAN runtime, as the main binary (python)
won't be linked against it. This will only work on Linux right now.
Differential Revision: https://reviews.llvm.org/D111004
* NFC but has some fixes for CMake glitches discovered along the way (things not cleaning properly, co-mingled depends).
* Includes previously unsubmitted fix in D98681 and a TODO to fix it more appropriately in a smaller followup.
Differential Revision: https://reviews.llvm.org/D101493