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.

toolchain/codegen/testdata/objcode/basic.carbon

@@ -3,8 +3,7 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 // ARGS: dump objcode --target_triple=x86_64-unknown-linux-gnu --output_file=%t %s
-// TODO: Support autoupdate with ARGS.
-// NOAUTOUPDATE
+// AUTOUPDATE
 // CHECK:STDOUT: Success: Object file is generated!
 
 fn Main() -> i32 { return 0; }