rustc: Further tweak linkage in ThinLTO

In #46382 the logic around linkage preservation with ThinLTO ws tweaked but the
loop that registered all otherwise exported GUID values as "don't internalize
me please" was erroneously too conservative and only asking "external" linkage
items to not be internalized. Instead we actually want the inversion of that
condition, everything *without* "local" linkage to be internalized.

This commit updates the condition there, adds a test, and...

Closes #46543

src/rustllvm/PassWrapper.cpp

@@ -970,7 +970,7 @@ LLVMRustCreateThinLTOData(LLVMRustThinLTOModule *modules,
   std::set<GlobalValue::GUID> ExportedGUIDs;
   for (auto &List : Ret->Index) {
     for (auto &GVS: List.second) {
-      if (!GlobalValue::isExternalLinkage(GVS->linkage()))
+      if (GlobalValue::isLocalLinkage(GVS->linkage()))
         continue;
       auto GUID = GVS->getOriginalName();
       if (!DeadSymbols.count(GUID))