[flang][windows] Run regression tests under Windows. NFCI.

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