[clang] [ItaniumCXXABI] Mark RTTI type name as global unnamed_addr (PR #111343)

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luxufan wrote:

> @luxufan I think we may need changes on the LLVM side before we can make progress here -- either to clarify what `unnamed_addr` is intended to mean, or perhaps to split it into separate attributes for cloning and merging.
> 
> But before we go down that path, this PR doesn't describe the motivation for this change, so it'd be useful to get an idea of how much of a difference you expect this to make. Are you seeing a lot of duplicated symbols containing type manglings?

@zygoloid  In our downstream project, we are trying to remove redundant RTTI data (type info and type name strings) in ThinLTO. To avoid some undefined symbols errors after elimination, I need to check the CanAutoHide flag https://github.com/llvm/llvm-project/blob/main/llvm/include/llvm/IR/ModuleSummaryIndex.h#L500 to decide if the symbol can be eliminated. The CanAutoHide flag is implied by function https://github.com/llvm/llvm-project/blob/main/llvm/include/llvm/IR/ModuleSummaryIndex.h#L500 which is partly implied by the unnamed_addr attribute. So if type info or type name string can be marked as unnamed_addr or local_unnamed_addr, it makes our implementation simple and correct.

Of course, mark as unnamed_addr or local_unnamed_addr is one of the solution, I can also add an extra flags in the module summary index to solve our problem.

https://github.com/llvm/llvm-project/pull/111343