This implements the proposed resolution of LWG 3446. I'm also adding
another new constrained specialization which isn't proposed by 3446, to
resolve the ambiguity when a type has both value_type and element_type
but denoting different types.
libstdc++-v3/ChangeLog:
* include/bits/iterator_concepts.h (indirectly_readable): Add
partial specializations to resolve ambiguities (LWG 3446).
* testsuite/24_iterators/associated_types/readable.traits.cc:
Check types with both value_type and element_type.
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