use one-bit state as discussed, and update test cases.

Re test coverage
- ThinLTO/X86/dot-dumpler.ll covers ModuleSummaryIndex::exportToDot
- Transforms/WholeProgramDevirt/import-indir.ll covers the update of
  FunctionSummaryTraits, and test both {def, decl}
- Assembler/thinlto-summary tests both {def, decl}
- The rest of test cases are backfill'ed with def and doesn't test decl.