libstdc++: Reduce size of std::bind_front(F) result

When there are no bound arguments to a std::bind_front call we don't
need the overhead of compiling, initializing, and accessing an empty
tuple.

libstdc++-v3/ChangeLog:

	* include/std/functional (_Bind_front0): New class template.
	(_Bind_front_t): Use _Bind_front0 when there are no bound
	arguments.
	* testsuite/20_util/function_objects/bind_front/107784.cc:
	New test.