libstdc++: Fix regression in std::uninitialized_fill for C++98 [PR120931]

libstdc++: Fix regression in std::uninitialized_fill for C++98 [PR120931]

A typo in r15-4473-g3abe751ea86e34 made it ill-formed to use
std::uninitialized_fill with iterators that aren't pointers (or pointers
wrapped in our __normal_iterator) if the value type is a narrow
character type.

libstdc++-v3/ChangeLog:

	PR libstdc++/120931
	* include/bits/stl_uninitialized.h (__uninitialized_fill<true>):
	Fix typo resulting in call to __do_uninit_copy instead of
	__do_uninit_fill.
	* testsuite/20_util/specialized_algorithms/uninitialized_fill/120931.cc:
	New test.