[lit] Fix a problem with spaces in the python path by adding quotes around it

Summary:
This fixes two failing tests on Windows with an installed version of python that has spaces in the path.

* elf/lto/cache.ll
* mach-o/dependency_info.yaml

Reviewers: zturner, llvm-commits, stella.stamenova

Subscribers: emaste, arichardson

Differential Revision: https://reviews.llvm.org/D43265

llvm-svn: 325650