Added the ability to get error strings back from failed

lldb_private::RegularExpression compiles and matches with:

    size_t
    RegularExpression::GetErrorAsCString (char *err_str, 
                                          size_t err_str_max_len) const;
    
Added the ability to search a variable list for variables whose names match
a regular expression:

    size_t
    VariableList::AppendVariablesIfUnique (const RegularExpression& regex, 
                                           VariableList &var_list, 
                                           size_t& total_matches);


Also added the ability to append a variable to a VariableList only if it is 
not already in the list:

    bool
    VariableList::AddVariableIfUnique (const lldb::VariableSP &var_sp);

Cleaned up the "frame variable" command:
- Removed the "-n NAME" option as this is the default way for the command to
  work.
- Enable uniqued regex searches on variable names by fixing the "--regex RE"
  command to work correctly. It will match all variables that match any
  regular expressions and only print each variable the first time it matches.
- Fixed the option type for the "--regex" command to by eArgTypeRegularExpression
  instead of eArgTypeCount

llvm-svn: 116178