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.
Nicholas Bishop
Richard Smith