[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.

Depends on D84855

Differential Revision: https://reviews.llvm.org/D84856