libstdc++: Fix handling of incomplete UTF-8 sequences in _Unicode_view

Eddie Nolan reported to me that _Unicode_view was not correctly
implementing the substitution of ill-formed subsequences with U+FFFD,
due to failing to increment the counter when the iterator reaches the
end of the sequence before a multibyte sequence is complete.  As a
result, the incomplete sequence was not completely consumed, and then
the remaining character was treated as another ill-formed sequence,
giving two U+FFFD characters instead of one.

To avoid similar mistakes in future, this change introduces a lambda
that increments the iterator and the counter together. This ensures the
counter is always incremented when the iterator is incremented, so that
we always know how many characters have been consumed.

libstdc++-v3/ChangeLog:

	* include/bits/unicode.h (_Unicode_view::_M_read_utf8): Ensure
	count of characters consumed is correct when the end of the
	input is reached unexpectedly.
	* testsuite/ext/unicode/view.cc: Test incomplete UTF-8
	sequences.