[flang] Add -h as a synonym for help

[richard.barton.arm]

Summary

As expected by user in http://lists.llvm.org/pipermail/flang-dev/2020-June/000404.html

Depends on D84856

flang/test/Driver/help.f90

+! RUN: %f18 -h 2>&1 | FileCheck %s
 ! 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()) {
         }
         args.pop_front();
       }
       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 == "-?") {
+    } else if (arg == "-h" || 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"