ParamIterator: recognize binary operators surrounded with spaces

Currently ParamIterator is used in 3 places. The `source` argument of
ParamIterator() is one of the following:
1) a return value of clang_getTypeSpelling() => formatted by libclang;
2) a user-formatted (arbitrarily formatted) macro definition, which
   cannot contain '<' or '>' within the parameter-list parentheses;
3) the `id` argument of Identifier(), which usually comes from and is
   formatted by libclang. Hopefully all identifiers that contain '<' are
   formatted by libclang.