Add missing standard library header inclusions (#5316)

Discovered by clang-tidy.

testing/base/capture_std_streams.cpp

@@ -8,6 +8,7 @@
 
 #include <fstream>
 #include <sstream>
+#include <string>
 
 #include "common/ostream.h"
 

testing/base/file_helpers.cpp

@@ -9,6 +9,7 @@
 #include <cstdlib>
 #include <fstream>
 #include <sstream>
+#include <string>
 #include <system_error>
 
 namespace Carbon::Testing {

testing/base/global_exe_path.cpp

@@ -4,6 +4,7 @@
 
 #include "testing/base/global_exe_path.h"
 
+#include <optional>
 #include <string>
 
 #include "common/check.h"

testing/base/source_gen.cpp

@@ -4,7 +4,11 @@
 
 #include "testing/base/source_gen.h"
 
+#include <algorithm>
+#include <array>
 #include <numeric>
+#include <string>
+#include <utility>
 
 #include "common/raw_string_ostream.h"
 #include "llvm/ADT/ArrayRef.h"

testing/base/source_gen_main.cpp

@@ -2,6 +2,9 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
+#include <optional>
+#include <system_error>
+
 #include "common/bazel_working_dir.h"
 #include "common/command_line.h"
 #include "common/init_llvm.h"

testing/base/source_gen_test.cpp

@@ -7,6 +7,8 @@
 #include <gmock/gmock.h>
 #include <gtest/gtest.h>
 
+#include <string>
+
 #include "common/set.h"
 #include "testing/base/global_exe_path.h"
 #include "toolchain/driver/driver.h"