Zcu: handle unreferenced test_functions correctly

Previously, `PerThread.populateTestFunctions` was analyzing the
`test_functions` declaration if it hadn't already been analyzed, so that
it could then populate it. However, the logic for doing this wasn't
actually correct, because it didn't trigger the necessary type
resolution. I could have tried to fix this, but there's actually a
simpler solution! If the `test_functions` declaration isn't referenced
or has a compile error, then we simply don't need to update it; either
it's unreferenced so its value doesn't matter, or we're going to get a
compile error anyway. Either way, we can just give up early. This avoids
doing semantic analysis after `performAllTheWork` finishes.

Also, get rid of the "Code Generation" progress node while updating the
test decl: this is a linking task.

src/Compilation.zig

@@ -2817,7 +2817,7 @@ pub fn update(comp: *Compilation, main_progress_node: std.Progress.Node) !void {
                 // The `test_functions` decl has been intentionally postponed until now,
                 // at which point we must populate it with the list of test functions that
                 // have been discovered and not filtered out.
-                try pt.populateTestFunctions(main_progress_node);
+                try pt.populateTestFunctions();
             }
 
             try pt.processExports();

src/Zcu/PerThread.zig

@@ -3229,39 +3229,42 @@ fn processExportsInner(
     }
 }
 
-pub fn populateTestFunctions(
-    pt: Zcu.PerThread,
-    main_progress_node: std.Progress.Node,
-) Allocator.Error!void {
+pub fn populateTestFunctions(pt: Zcu.PerThread) Allocator.Error!void {
     const zcu = pt.zcu;
     const gpa = zcu.gpa;
     const ip = &zcu.intern_pool;
+
+    // Our job is to correctly set the value of the `test_functions` declaration if it has been
+    // analyzed and sent to codegen, It usually will have been, because the test runner will
+    // reference it, and `std.builtin` shouldn't have type errors. However, if it hasn't been
+    // analyzed, we will just terminate early, since clearly the test runner hasn't referenced
+    // `test_functions` so there's no point populating it. More to the the point, we potentially
+    // *can't* populate it without doing some type resolution, and... let's try to leave Sema in
+    // the past here.
+
     const builtin_mod = zcu.builtin_modules.get(zcu.root_mod.getBuiltinOptions(zcu.comp.config).hash()).?;
     const builtin_file_index = zcu.module_roots.get(builtin_mod).?.unwrap().?;
-    pt.ensureFileAnalyzed(builtin_file_index) catch |err| switch (err) {
-        error.AnalysisFail => unreachable, // builtin module is generated so cannot be corrupt
-        error.OutOfMemory => |e| return e,
-    };
-    const builtin_root_type = Type.fromInterned(zcu.fileRootType(builtin_file_index));
-    const builtin_namespace = builtin_root_type.getNamespace(zcu).unwrap().?;
+    const builtin_root_type = zcu.fileRootType(builtin_file_index);
+    if (builtin_root_type == .none) return; // `@import("builtin")` never analyzed
+    const builtin_namespace = Type.fromInterned(builtin_root_type).getNamespace(zcu).unwrap().?;
+    // We know that the namespace has a `test_functions`...
     const nav_index = zcu.namespacePtr(builtin_namespace).pub_decls.getKeyAdapted(
         try ip.getOrPutString(gpa, pt.tid, "test_functions", .no_embedded_nulls),
         Zcu.Namespace.NameAdapter{ .zcu = zcu },
     ).?;
+    // ...but it might not be populated, so let's check that!
+    if (zcu.failed_analysis.contains(.wrap(.{ .nav_val = nav_index })) or
+        zcu.transitive_failed_analysis.contains(.wrap(.{ .nav_val = nav_index })) or
+        ip.getNav(nav_index).status != .fully_resolved)
     {
-        // We have to call `ensureNavValUpToDate` here in case `builtin.test_functions`
-        // was not referenced by start code.
-        zcu.sema_prog_node = main_progress_node.start("Semantic Analysis", 0);
-        defer {
-            zcu.sema_prog_node.end();
-            zcu.sema_prog_node = std.Progress.Node.none;
-        }
-        pt.ensureNavValUpToDate(nav_index) catch |err| switch (err) {
-            error.AnalysisFail => return,
-            error.OutOfMemory => return error.OutOfMemory,
-        };
+        // The value of `builtin.test_functions` was either never referenced, or failed analysis.
+        // Either way, we don't need to do anything.
+        return;
     }
 
+    // Okay, `builtin.test_functions` is (potentially) referenced and valid. Our job now is to swap
+    // its placeholder `&.{}` value for the actual list of all test functions.
+
     const test_fns_val = zcu.navValue(nav_index);
     const test_fn_ty = test_fns_val.typeOf(zcu).slicePtrFieldType(zcu).childType(zcu);
 
@@ -3363,17 +3366,8 @@ pub fn populateTestFunctions(
         } });
         ip.mutateVarInit(test_fns_val.toIntern(), new_init);
     }
-    {
-        assert(zcu.codegen_prog_node.index == .none);
-        zcu.codegen_prog_node = main_progress_node.start("Code Generation", 0);
-        defer {
-            zcu.codegen_prog_node.end();
-            zcu.codegen_prog_node = std.Progress.Node.none;
-        }
-
-        // The linker thread is not running, so we actually need to dispatch this task directly.
-        @import("../link.zig").linkTestFunctionsNav(pt, nav_index);
-    }
+    // The linker thread is not running, so we actually need to dispatch this task directly.
+    @import("../link.zig").linkTestFunctionsNav(pt, nav_index);
 }
 
 /// Stores an error in `pt.zcu.failed_files` for this file, and sets the file