I discovered this when experimenting with `std::map::iterator`, which has a const overload of `operator*` that returns a non-const reference, and does not have a const overload of `operator*`.
rdar://112471779
This fixes the automatic `std::unordered_map` conformance to CxxDictionary on Linux. Previously `std::unordered_map::const_iterator` was not auto-conformed to UnsafeCxxInputIterator because its `operator==` is defined on a templated base class of `const_iterator`.
rdar://105220600
To determine whether to conform a C++ type to `CxxSequence` protocol automatically, ClangImporter checks if the corresponding iterator type conforms to `UnsafeCxxInputIterator`.
This logic had false-positives, e.g. `Optional<OpaquePointer>` was treated as if it conforms to `UnsafeCxxInputIterator` while it actually doesn't. This happened because `lookupConformance` returned a conformance with a conditional requirement (`Wrapped : UnsafeCxxInputIterator`) that is not satisfied for `OpaquePointer`.
rdar://100265664
This makes ClangImporter automatically conform C++ sequence types to `Cxx.CxxSequence` protocol.
We consider a C++ type to be a sequence type if it defines `begin()` & `end()` methods that return iterators of the same type which conforms to `UnsafeCxxInputIterator`.
This teaches ClangImporter to synthesize conformances of C++ iterator types to `UnsafeCxxInputIterator` protocol from the `Cxx` module.
We consider a C++ type to be an iterator if it defines a subtype (usually a typedef or a using decl) called `iterator_category` that inherits from `std::input_iterator_tag`.
rdar://96235368
This change adds basic helper protocols and structs that are going to be used for making C++ sequences and collection safe and Swifty by adding conformances to `Swift.Sequence`, `Swift.Collection`, etc.
This is not meant to be a final design.
