Revert "[lit] Avoid os.path.realpath in lit.py due to MAX_PATH limitations on Windows"

This reverts commit c1cf459c.

Reverting to permit time to explore the underlying issue.  This change
regressed the clang-PPC64-AIX and m68k-linux-cross builders.

Differential Revision: https://reviews.llvm.org/D153138
Reviewed By: compnerd