[ELF] Internalize enum

g++ -flto has a diagnostic `-Wodr` about mismatched redeclarations,
which even apply to `enum`.

Fix #83529

Reviewers: thesamesam

Reviewed By: thesamesam

Pull Request: https://github.com/llvm/llvm-project/pull/83604

(cherry picked from commit 4a3f7e79)