[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

(cherry picked from commit b068d19)

flang/test/Driver/help.f90

@@ -0,0 +1,9 @@
+! RUN: %f18 -help 2>&1 | FileCheck %s
+! RUN: %f18 --help 2>&1 | FileCheck %s
+! RUN: %f18 -? 2>&1 | FileCheck %s
+
+! CHECK: f18: LLVM Fortran compiler
+
+! CHECK:   -help                print this again
+! CHECK: Unrecognised options are passed through to the external compiler
+! CHECK: set by F18_FC (see defaults).

flang/tools/f18/f18.cpp

@@ -602,6 +602,19 @@ int main(int argc, char *const argv[]) {
       driver.getSymbolsSources = true;
     } else if (arg == "-help" || arg == "--help" || arg == "-?") {
       llvm::errs()
+          << "f18: LLVM Fortran compiler\n"
+          << "\n"
+          << "Usage: f18 [options] <input files>\n"
+          << "\n"
+          << "Defaults:\n"
+          << "  When invoked with input files, and no options to tell\n"
+          << "  it otherwise, f18 will unparse its input and pass that on to an\n"
+          << "  external compiler to continue the compilation.\n"
+          << "  The external compiler is specified by the F18_FC environment\n"
+          << "  variable. The default is 'gfortran'.\n"
+          << "  If invoked with no input files, f18 reads source code from\n"
+          << "  stdin and runs with -fdebug-measure-parse-tree -funparse.\n"
+          << "\n"
           << "f18 options:\n"
           << "  -Mfixed | -Mfree | -ffixed-form | -ffree-form   force the "
              "source form\n"
@@ -635,7 +648,8 @@ int main(int argc, char *const argv[]) {
           << "  -fget-symbols-sources\n"
           << "  -v -c -o -I -D -U    have their usual meanings\n"
           << "  -help                print this again\n"
-          << "Other options are passed through to the compiler.\n";
+          << "Unrecognised options are passed through to the external compiler\n"
+          << "set by F18_FC (see defaults).\n";
       return exitStatus;
     } else if (arg == "-V") {
       llvm::errs() << "\nf18 compiler (under development)\n";