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.
Depends on D84855
Differential Revision: https://reviews.llvm.org/D84856