[Clang] avoid assertion in __underlying_type for enum redeclarations (#177984)
Fixes #177943
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This patch addresses cases where `__underlying_type` is used with enum
redeclarations. The previously added assertion
(https://github.com/llvm/llvm-project/pull/155900) treated a missing
`int` on the referenced `EnumDecl` as an indicator of a _demoted
definition_, while this condition can also occur for redeclarations.