This fixed is based on the assert in LinkageComputer::getLVForDecl(…) which assumes that all the decls in a redecl chain have the same linkage.
This change was trigged by a bug that came up when debugging llc and running the following expression in while in `SelectionDAG::getNode(…)
p VT.getVectorElementType() == Operand.getValueType()
Evaluating this expression leads to import of an operator== for I believe a std::set iterator. One with external linkage and one with unique external linkage but they end in the same redecl chain which triggers the assert in LinkageComputer::getLVForDecl(…).
This case has proven difficult to reduce to a minimal test case.