[clang] [clang-tools-extra] [clang][NFC] Refactor `clang::Linkage` (PR #71049)

[Vlad Serebrennikov via cfe-commits]

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@@ -2214,7 +2214,7 @@ void ASTWriter::WriteDeclAbbrevs() {
   Abv->Add(BitCodeAbbrevOp(0));                       // TSCSpec
   Abv->Add(BitCodeAbbrevOp(0));                       // InitStyle
   Abv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 1)); // isARCPseudoStrong
-  Abv->Add(BitCodeAbbrevOp(0));                       // Linkage
+  Abv->Add(BitCodeAbbrevOp(1));                         // Linkage
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Endilll wrote:

@ChuanqiXu9 confirmed that touching this is fine (thank you!). If I guess correctly that this value corresponds to `Linkage::None` we have in some default constructors, we should consider changing that default to `Linkage::Invalid`.

`Linkage::None` being default feels like we computed the linkage and arrived at `Linkage::None` even when it's not the case.

CC @AaronBallman 

https://github.com/llvm/llvm-project/pull/71049