stage2: fix small memory leak of test_functions when using zig test

The way `zig test` works is that it uses a stand-in

var test_functions: []const TestFn = undefined;

during semantic analysis, but then just before codegen, it swaps out the
value with a constant like this:

const test_functions: []const TestFn = .{foo, bar, baz, etc};

Before this commit, the `Module.Variable` associated with the stand-in
value was leaked; now it is properly cleaned up before being replaced.

src/Module.zig

@@ -4767,15 +4767,25 @@ pub fn populateTestFunctions(mod: *Module) !void {
     try mod.linkerUpdateDecl(array_decl);
 
     {
-        var arena_instance = decl.value_arena.?.promote(gpa);
-        defer decl.value_arena.?.* = arena_instance.state;
-        const arena = &arena_instance.allocator;
+        var new_decl_arena = std.heap.ArenaAllocator.init(gpa);
+        errdefer new_decl_arena.deinit();
+        const arena = &new_decl_arena.allocator;
 
-        decl.ty = try Type.Tag.const_slice.create(arena, try tmp_test_fn_ty.copy(arena));
-        decl.val = try Value.Tag.slice.create(arena, .{
+        // This copy accesses the old Decl Type/Value so it must be done before `clearValues`.
+        const new_ty = try Type.Tag.const_slice.create(arena, try tmp_test_fn_ty.copy(arena));
+        const new_val = try Value.Tag.slice.create(arena, .{
             .ptr = try Value.Tag.decl_ref.create(arena, array_decl),
             .len = try Value.Tag.int_u64.create(arena, mod.test_functions.count()),
         });
+
+        // Since we are replacing the Decl's value we must perform cleanup on the
+        // previous value.
+        decl.clearValues(gpa);
+        decl.ty = new_ty;
+        decl.val = new_val;
+        decl.has_tv = true;
+
+        try decl.finalizeNewArena(&new_decl_arena);
     }
     try mod.linkerUpdateDecl(decl);
 }

src/Sema.zig

@@ -10762,6 +10762,11 @@ fn zirVarExtended(
     }
 
     const new_var = try sema.gpa.create(Module.Var);
+
+    log.debug("created variable {*} owner_decl: {*} ({s})", .{
+        new_var, sema.owner_decl, sema.owner_decl.name,
+    });
+
     new_var.* = .{
         .owner_decl = sema.owner_decl,
         .init = init_val,