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

[Vlad Serebrennikov via cfe-commits]

================
@@ -84,22 +88,33 @@ inline bool isUniqueGVALinkage(GVALinkage L) {
 }
 
 inline bool isExternallyVisible(Linkage L) {
-  return L >= VisibleNoLinkage;
+  switch (L) {
+  case Linkage::Invalid:
+    llvm_unreachable("Linkage hasn't been computed!");
----------------
Endilll wrote:

@AaronBallman I sprinkled a lot of `llvm_unreachable` in this patch, following an example I've seen in one of the switches on `Linkage`. Let me know if you'd like this to be handled in a different way.

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