lit: modernize the lit configuration for the lit tests

Summary: This also normalizes the config feature that represents the windows platform to "system-windows" as opposed to having both "windows" and "system-windows"

Reviewers: asmith, probinson

Subscribers: delcypher, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D61798

llvm-svn: 361998