[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 change updates several configuration files which specify the path to python as a substitution and also remove quotes from existing tests.

Reviewers: asmith, zturner, alexshap, jakehehrlich

Reviewed By: zturner, alexshap, jakehehrlich

Subscribers: mehdi_amini, nemanjai, eraman, kbarton, jakehehrlich, steven_wu, dexonsmith, stella.stamenova, delcypher, llvm-commits

Differential Revision: https://reviews.llvm.org/D50206

llvm-svn: 339073