[Bash-completion] Fixed a bug that file doesn't autocompleted after =

[yamaguchi]

Summary

File path wasn't autocompleted after -fmodule-cache-path=[tab], so
fixed this bug by checking if $flags contains only a newline or not.

[ruiu]

LGTM

cfe/trunk/utils/bash-autocomplete.sh

_clang()
   flags=$( "$path" --autocomplete="$arg" 2>/dev/null )
   # If clang is old that it does not support --autocomplete,
   # fall back to the filename completion.
   if [[ "$?" != 0 ]]; then
     _clang_filedir
     return
   fi
 
-  if [[ "$cur" == '=' ]]; then
-    COMPREPLY=( $( compgen -W "$flags" -- "") )
-  elif [[ "$flags" == "" || "$arg" == "" ]]; then
+  # When clang does not emit any possible autocompletion, or user pushed tab after " ",
+  # just autocomplete files.
+  if [[ "$flags" == "$(echo -e '\n')" || "$arg" == "" ]]; then
+    # If -foo=<tab> and there was no possible values, autocomplete files.
+    [[ "$cur" == '=' || "$cur" == -*= ]] && cur=""
     _clang_filedir
+  elif [[ "$cur" == '=' ]]; then
+    COMPREPLY=( $( compgen -W "$flags" -- "") )
   else
     # Bash automatically appends a space after '=' by default.
     # Disable it so that it works nicely for options in the form of -foo=bar.
     [[ "${flags: -1}" == '=' ]] && compopt -o nospace 2> /dev/null
     COMPREPLY=( $( compgen -W "$flags" -- "$cur" ) )
   fi
 }
 complete -F _clang clang