[cxx-interop] Support importing value types exported from Swift
Whenever a Swift type is exported to C++, we create a corresponding type on the
C++ side. However, we did not support importing value types back into Swift.
This PR addresses this problem.
Unfortunately, we cannot just set the C++ type as the ClangNode of
Swift's type. Types with ClangNodes are using the foreign
representation, but in this case we want to use the native type whenever
possible (e.g., for codegen). To work this around, this PR updates
ClangTypeConverter to make sure we have the correct Clang types when we
emit function calls to C++ APIs. It also updates the importer to not
attempt to import types that are annotated with ExternalSourceSymbolAttr
attributes as Swift types.
