Fixes link failures when referencing a symbol involving a fingerprint
from a different package.
Previously we included the `Namespace`'s `import_id` as part of its
fingerprint, which caused local and imported namespaces to get different
fingerprints. We now store the `import_id` on the `NameScope` instead of
on the `Namespace` inst to avoid this problem.
Also, when we reach a package-level `NameScopeId`, consistently
fingerprint it as a (package name, library name) pair. Previously the
fingerprinting depended on whether it was imported or not, as an
imported `NameScopeId` had a parent scope (the current package). We need
to include the library name here so that private entities with the same
name in different libraries have different fingerprints.
* For Carbon `base class C`, export as a regular C++ class.
* For Carbon `class C`, export with the C++ `final` keyword attribute.
* For Carbon `abstract C`, mark the destructor as pure virtual in cases
where no member function is abstract, or emit an error if the destructor
is not virtual.
To support the final point, mark the destructor of an exported class as
virtual if it overrides a virtual destructor from the base class.
In passing, fix a crash exporting fields if the class has an invalid
base type.
A destructor is added to the C++ class definition in
`CarbonExternalASTSource::CompleteType`. The destructor calls a Carbon
function that calls the `Destroy` operator.
The FunctionDecl created for calling the Carbon thunk now takes a `self`
parameter for non-static methods, and the C++ thunk now passes an extra
argument for that `self` parameter when needed.
The CXXMethodDecl thunk created for calling methods now sets the storage
class appropriate depending on whether the method is static or not.
To reduce the number of parameters being passed around to thunk-building
functions, added a `FunctionInfo` struct and pass that around instead.
We don't yet populate the bases or fields, so the class types show up as
empty classes in C++ for now. But we do allow calls to static member
functions.
Richard Smith
Nicholas Bishop