Allow the lit test suite to run under Windows. This encompasses the following changes:
* Define `lit_tools_dir` for flang's test configuration
* Replace `(<command> || true)` idiom with `not <command>`
* Add `REQUIRES: shell` on tests that invoke a shell script
Reviewed By: awarzynski
Differential Revision: https://reviews.llvm.org/D89368
58c3f20bbf