When flang is invoked with no files it waits for input on stdin. Make it
print a message saying this to prevent the user being surprised.
[flang] Add details to --help screen on default behaviour
Add a usage string and a defaults section that clarifies:
- If no input files are given, f18 reads from stdin
- If no input files are given, f18 dumps the parse tree.
- The default behaviour is to exec F18_FC.
- The fefault F18_FC setting is 'gfortran'
Adds a simple regression test which tests the top and tail of the help
screen and the exit status.
[flang] Add -h as a synonym for help
As expected by user in http://lists.llvm.org/pipermail/flang-dev/2020-June/000404.html