libstdc++: Fix concept checks for iterators

This adds some additional checks the the C++98-style concept checks for
iterators, and removes some bogus checks for mutable iterators. Instead
of requiring that the result of dereferencing a mutable iterator is
assignable (which is a property of the value type, not required for the
iterator) check that the reference type is a non-const reference to the
value type.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>

libstdc++-v3/ChangeLog:

	* include/bits/boost_concept_check.h (_ForwardIteratorConcept)
	(_BidirectionalIteratorConcept, _RandomAccessIteratorConcept):
	Check result types of iterator operations.
	(_Mutable_ForwardIteratorConcept): Check that iterator's
	reference type is a reference to its value type.
	(_Mutable_BidirectionalIteratorConcept): Do not require the
	value type to be assignable.
	(_Mutable_RandomAccessIteratorConcept): Likewise.
	* testsuite/24_iterators/operations/prev_neg.cc: Adjust dg-error
	line number.

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