[ODRHash][NFC] Add missing 'select' case for `ODRMismatchDecl`.

No test changes because `err_module_odr_violation_mismatch_decl_unknown`
is a catch-all when custom diagnostic is missing. And missing custom
diagnostic we should fix by implementing it, not by improving the
general case. But if we pass enum value not covered by 'select', clang
can crash, so protect against that.

Differential Revision: https://reviews.llvm.org/D126566