This makes ClangImporter automatically conform C++ sequence types to `Cxx.UnsafeCxxInputIterator` protocol.
We consider a C++ type to be a random access iterator type if conforms to `UnsafeCxxInputIterator`, and additionally defines `operator-` and `operator+=`.
This lifts the requirement for the user to explicitly add `import Cxx` in Swift code that relies on protocols from the `Cxx` module, such as `CxxSequence`.
These errors were sometimes produced when synthesizing conformance to `UnsafeCxxInputIterator`:
```
<unknown>:0: error: circular reference
<unknown>:0: note: through reference here
<unknown>:0: note: through reference here
```
This happened because `NominalTypeDecl::lookupDirect` attempts to deserialize Swift extensions of the type, which might belong to the module which has a dependency to the module that is currently being imported, which leads to deserialization errors.
This change makes sure we don't call `NominalTypeDecl::lookupDirect` when synthesizing conformances for C++ types.
Previosly we didn't detect `func ==` that was declared out-of-class when synthesizing conformaces to `UnsafeCxxInputIterator`. Now we do.
rdar://96235368
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
