Types without constructors do not gain a make_unique

[adetaylor]

If.a struct/class has a constructor, we generate a make_unique associated function. We don't do that for types which have no constructors. We (probably?) should at least for non-POD types, calling the default C++ constructor.

This may be difficult depending on whether we can find out that the type has a private non-default constructor.

[adetaylor]

I've fixed up this PR, but I don't want to merge it until I've investigated the vexed issue of private constructors.

[martinboehme]

Just adding some notes here.

I think the most robust way to deal with this would be:

• Provide access to Clang's CXXRecordDecl::needsImplicitDefaultConstructor() in libclang (I think this isn't available today)
• Have bindgen call that API and output a suitable attribute on the generated Rust struct
• Interpret this attribute in autocxx

If libclang doesn't want to expose needsImplicitDefaultConstructor(), then bindgen could try to replicate the logic that governs whether a class has an implicit default constructor, but that seems potentially error-prone.

For reference, here is how Swift's C++ interop deals with this:

https://github.com/apple/swift/blob/main/lib/ClangImporter/ImportDecl.cpp#L3685