[clang] [clang][RelativeVTables] Make the rtti_proxy LinkOnceODR instead of External linkage (PR #67755)

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PiJoules wrote:

This can be landed independently of https://github.com/llvm/llvm-project/pull/67754 but we'll really only see it's intended use with https://github.com/llvm/llvm-project/pull/67754.

https://github.com/llvm/llvm-project/pull/67755