Add unittest for covering ErrorBuilder implicit cast and metaprogramming (#2882)

- update unittest of Error, cover the casting of ErrorBuilder.
- add unittest of metaprogramming.h

common/metaprogramming_test.cpp

@@ -0,0 +1,32 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include "common/metaprogramming.h"
+
+#include <gtest/gtest.h>
+
+#include <string>
+
+#include "llvm/Support/raw_ostream.h"
+
+namespace Carbon::Testing {
+namespace {
+
+TEST(MetaProgrammingTest, RequiresBasic) {
+  bool result = Requires<int, int>([](int a, int b) { return a + b; });
+  EXPECT_TRUE(result);
+}
+
+struct TypeWithPrint {
+  void Print(llvm::raw_ostream& os) const { os << "Test"; }
+};
+
+TEST(MetaProgrammingTest, RequiresPrintMethod) {
+  bool result = Requires<const TypeWithPrint, llvm::raw_ostream>(
+      [](auto&& t, auto&& out) -> decltype(t.Print(out)) {});
+  EXPECT_TRUE(result);
+}
+
+}  // namespace
+}  // namespace Carbon::Testing