[BOLT][UTILS] Usability improvements for nfc-check-setup

# Stash local changes before checkout.
# Print a message that the source repository revision has been changed, with
  instructions to switch back.
# Make the script executable.
# Print sample instructions how to run bolt tests.
# Assume that llvm-bolt-wrapper script is in the same source directory.

Reviewed By: rafauler

Differential Revision: https://reviews.llvm.org/D126941