[lit] Fix 42812: lit test suite can no longer be run stand-alone
This change updates the lit.cfg file to use llvm_config when it is available, but when it is not, it directly modifies the config object. This makes it possible to run the lit tests standalone without having built llvm (as long as the correct binaries are present in the path such as FileCheck and not).
Because the lit tests don't take a hard dependency on llvm_config, some features such as system-windows have to have definitions in lit's cfg file as well. This is a potential issue as the os features sometimes change names (for example, we went from windows to system-windows, etc.). This can cause drift between lit's tests and the rest of the llvm tests.
Reviewers: probinson, mgorny
Reviewed By: mgorny
Subscribers: delcypher, llvm-commits, asmith
Differential Revision: https://reviews.llvm.org/D65674