[flang] Add -h as a synonym for help

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

Depends on D84856

Differential Revision: https://reviews.llvm.org/D84857

(cherry picked from commit 30e45f3)

Author: RichBarton-Arm

flang/test/Driver/help.f90

@@ -1,3 +1,4 @@
+! 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

flang/tools/f18/f18.cpp

@@ -600,7 +600,7 @@ int main(int argc, char *const argv[]) {
       driver.getDefinitionArgs = {arguments[0], arguments[1], arguments[2]};
     } else if (arg == "-fget-symbols-sources") {
       driver.getSymbolsSources = true;
-    } else if (arg == "-help" || arg == "--help" || arg == "-?") {
+    } else if (arg == "-h" || arg == "-help" || arg == "--help" || arg == "-?") {
       llvm::errs()
           << "f18: LLVM Fortran compiler\n"
           << "\n"