[lit] Fix problem in how Python versions open files with different encodings

Summary:
This issue was found when running the clang unit test on Windows. Python 3.x cannot open some of the files that the tests are using with a simple open because of their encoding. Python 2.7+ and Python 3.x both support io.open which allows for an encoding to be specified. 

This change will determine whether two files being compared should be opened (and then compared) as text or binary and whether to use utf-8 or the default encoding before proceeding with a line-by-line comparison.

Patch by Stella Stamenova!

Reviewers: zturner, llvm-commits, rnk, MaggieYi

Reviewed By: zturner

Subscribers: asmith, MatzeB, stella.stamenova, delcypher, llvm-commits

Differential Revision: https://reviews.llvm.org/D43165

llvm-svn: 329012