[cxx-interop] [NFC] Remove unused ClangImporter::Implementation pointer
This was previously used to control whether a CxxRecordSemantics or
CxxRecordAsSwiftType request would trigger diagnostics when determining
a type is a foreign reference type. That interface led to obscure
control flow and was removed recently. This patch removes the vestigial
pointer field those requests used.
(Actually, that pointer was *never* used by CxxRecordAsSwiftType
requests; its call sites just happened to pick it up because it shares
the same input descriptor type as CxxRecordSemantics).
rdar://170858418