[bash-autocompletion] Fix bug when a flag ends with '='

There was a bug that when a flag ends with '=' and no value was suggested,
clang autocompletes the flag itself.
For example, in bash, it looked like this:
```
$ clang -fmodule-file=[tab]
-> $clang -fmodule-file=-fmodule-file
```
This is not what we expect. We expect a file autocompletion when no value
was found. With this patch, pressing tab suggests files in the current
directory.

Reviewers: teemperor, ruiu

Subscribers: cfe-commits
llvm-svn: 345121