[lit, python] Always add quotes around the python path in lit

Summary:
The issue with the python path is that the path to python on Windows can contain spaces. To make the tests always work, the path to python needs to be surrounded by quotes.

This is a companion change to: https://reviews.llvm.org/D50206

Reviewers: asmith, zturner

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D50281

llvm-svn: 339074