Sema: fix @extern decls

Closes #18550

src/codegen/llvm.zig

@@ -2941,9 +2941,13 @@ pub const Object = struct {
         const target = owner_mod.resolved_target.result;
         const sret = firstParamSRet(fn_info, zcu);
 
+        const is_extern = decl.isExtern(zcu);
         const function_index = try o.builder.addFunction(
             try o.lowerType(zig_fn_type),
-            try o.builder.string(ip.stringToSlice(try decl.getFullyQualifiedName(zcu))),
+            try o.builder.string(ip.stringToSlice(if (is_extern)
+                decl.name
+            else
+                try decl.getFullyQualifiedName(zcu))),
             toLlvmAddressSpace(decl.@"addrspace", target),
         );
         gop.value_ptr.* = function_index.ptrConst(&o.builder).global;
@@ -2951,7 +2955,6 @@ pub const Object = struct {
         var attributes: Builder.FunctionAttributes.Wip = .{};
         defer attributes.deinit(&o.builder);
 
-        const is_extern = decl.isExtern(zcu);
         if (!is_extern) {
             function_index.setLinkage(.internal, &o.builder);
             function_index.setUnnamedAddr(.unnamed_addr, &o.builder);