Fixed the "expression" command when it comes to using it with the new GDB format

command suffix:

(lldb) expression/x 3+3

Since "expression" is a raw command that has options, we need to make sure the
command gets its options properly terminated with a "--".

Also fixed an issue where if you try to use the GDB command suffix on a 
command that doesn't support the "--gdb-format" command, it will report an
appropriate error.

For the fix above, you can query an lldb_private::Options object to see if it
supports a long option by name.

llvm-svn: 143266