[flang] Add details to --help screen on default behaviour

[richard.barton.arm]

Summary

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

[sscalpone]

Hi Rich, sorry not to have replied to your other review. No one on the NV team needs f18 to read from stdin.

flang/test/Driver/help.f90

+! 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

if (arg.empty()) {
       }
       driver.getDefinitionArgs = {arguments[0], arguments[1], arguments[2]};
     } else if (arg == "-fget-symbols-sources") {
       driver.getSymbolsSources = true;
     } else if (arg == "-byteswapio") {
       driver.byteswapio = true; // TODO: Pass to lowering, generate call
     } 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"
           << "  -Mextend | -ffixed-line-length-132   132-column fixed form\n"
           << "  -f[no-]backslash     enable[disable] \\escapes in literals\n"
           << "  -M[no]backslash      disable[enable] \\escapes in literals\n"
           << "  -Mstandard           enable conformance warnings\n"
           << "  -std=<standard>      enable conformance warnings\n"
if (arg.empty()) {
           << "  -fdebug-dump-symbols\n"
           << "  -fdebug-resolve-names\n"
           << "  -fdebug-instrumented-parse\n"
           << "  -fdebug-no-semantics  disable semantic checks\n"
           << "  -fget-definition\n"
           << "  -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";
       return exitStatus;
     } else {
       driver.F18_FCArgs.push_back(arg);
       if (arg == "-v") {
         driver.verbose = true;