[flang][driver] Add support for `--version` in the bash wrapper

[awarzynski]

Summary

The bash wrapper script, flang, calls flang-new -fc1 under the hood,
which does not support --version (this is consistent with `clang -cc1
--version`). This change is needed for flang --version to work as
expected.

Note that flang --version (the Flang bash wrapper script for the
compiler driver) gives rather minimal output compared to `flang-new
--version` (the Flang compiler driver). As the wrapper script is just a
temporary solution for us, this should be sufficient.

[DavidSpickett]

LGTM

flang/tools/f18/flang.in

 INPUT_FILES=()
 OPTIONS=()
 OUTPUT_FILE=""
 MODULE_DIR=""
 INTRINSICS_MOD_DIR=""
 COMPILE_ONLY="False"
 PREPROCESS_ONLY="False"
 TEMP_OUTPUT="flang_temp_out_"
+PRINT_VERSION="False"
 
 # === parse_args ==============================================================
 #
 # Parse the input arguments passed to this script. Sets the global variables
 # declared at the top.
 #
 # INPUTS:
 #   $1 - all input arguments
while [ "${1:-}" != "" ]; do
         if [[ $1 == "-c" ]]; then
           COMPILE_ONLY="True"
         fi
 
         if [[ $1 == "-E" ]]; then
           PREPROCESS_ONLY="True"
         fi
 
+        if [[ $1 == "-v" || $1 == "--version" ]]; then
+          PRINT_VERSION="True"
+        fi
+
         shift
         continue
 
       # CASE 2: A regular file (either source or a library file)
       elif [[ -f "$1" ]]; then
         INPUT_FILES+=($1)
         shift
         continue
if
       [[ $opt == "-fopenacc" ]] ||
       [[ $opt == "-fopenmp" ]] ||
       [[ $opt == "-fxor-operator" ]] ||
       [[ $opt == "-help" ]] ||
       [[ $opt == "-nocpp" ]] ||
       [[ $opt == "-pedantic" ]] ||
       [[ $opt =~ ^-std=.* ]] ||
       [[ $opt =~ ^-U.* ]] ||
-      [[ $opt == "--version" ]] ||
       [[ $opt == "-Werror" ]]; then
       flang_opts+=($opt)
     elif [[ $opt =~ -I.* ]] || [[ $opt =~ -J.* ]]; then
       # Options that are needed for both Flang and the external driver.
       flang_opts+=($opt)
       fc_opts+=($opt)
     else
       # All other options are claimed for the external driver.

 }
 
 # === main ====================================================================
 # Main entry point for this script
 # =============================================================================
 main() {
   parse_args "$@"
 
+  if [[ $PRINT_VERSION == "True" ]]; then
+    echo "flang version @FLANG_VERSION@"
+    exit 0
+  fi
+
   fortran_source_files=()
   other_source_files=()
   object_files=()
   lib_files=()
   categorise_files INPUT_FILES fortran_source_files other_source_files object_files lib_files
 
   if [[ $PREPROCESS_ONLY == "True" ]]; then
     preprocess fortran_source_files other_source_files OPTIONS