[asan] Quote the path to the Python exe in case it has spaces

These days, Python 3 installs itself into Program Files, so it often has
spaces. At first, I resisted this, and I reinstalled it globally into
C:/Python37, similar to the location used for Python 2.7. But then I
updated VS 2019, and it uninstalled my copy of Python and installed a
new one inside "C:/Program Files (x86)/Microsoft Visual Studio/". At
this point, I gave up and switched to using its built-in version of
Python. However, now these tests fail, and have to be made aware of the
possibility of spaces in paths. :(

llvm-svn: 364077