With the change to include `SmallVector.h` directly in `LLVM.h` rather
than forward declaring in the only case it matters (ie. Clang <= 5),
these fixes are no longer needed. Since defaulted version is preferred
when there's no better choice (which is presumably the case if that's
how they were originally added), use it instead. Some uses were instead
changed to add `llvm::` so remove that too.
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
