Implement autoupdate for file_test. (#3043)

I've migrated the toolchain autoupdate scripts here, I just need a
little more time to do the explorer side (which I need to check
performance, that may require multithreading as we do in the current
script). However, this felt substantial enough to share and it means I
can handle autoupdate in more of the toolchain, including preparatory
work for autoupdate on multi-file tests.

Once explorer is done I'll remove the old script.

testing/file_test/testdata/args.carbon

@@ -3,4 +3,5 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
 // ARGS: abc file=%t %s
+// AUTOUPDATE
 // CHECK:STDOUT: 3 args: `abc`, `file={{.+}}/temp_file`, `args.carbon`

testing/file_test/testdata/example.carbon

@@ -2,10 +2,12 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
+// Can't autoupdate due to the `{{.+}}` check.
+// NOAUTOUPDATE
 // CHECK:STDOUT: 2 args: `default_args`, `example.carbon`
 // CHECK:STDOUT: something
 // CHECK:STDOUT:
-// CHECK:STDOUT: [[@LINE+1]]: Line delta
-// CHECK:STDOUT: [[@LINE-1]]: Negative line delta
+// CHECK:STDOUT: example.carbon:[[@LINE+1]]: Line delta
+// CHECK:STDOUT: example.carbon:[[@LINE-1]]: Negative line delta
 // CHECK:STDOUT: +*[]{}
 // CHECK:STDOUT: F{{.+}}z

testing/file_test/testdata/fail_example.carbon

@@ -2,5 +2,6 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
+// AUTOUPDATE
 // CHECK:STDOUT: 2 args: `default_args`, `fail_example.carbon`
 // CHECK:STDERR: Oops

testing/file_test/testdata/two_files.carbon

@@ -2,10 +2,13 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
+// AUTOUPDATE
 // CHECK:STDOUT: 3 args: `default_args`, `a.carbon`, `b.carbon`
 
 // --- a.carbon
-// CHECK:STDOUT: a.carbon: [[@LINE+0]]
+// CHECK:STDOUT: a.carbon:[[@LINE+1]]: 1
+aaa
 
 // --- b.carbon
-// CHECK:STDOUT: b.carbon: [[@LINE+1]]
+// CHECK:STDOUT: b.carbon:[[@LINE+1]]: 2
+bbb