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/assembly/fail_target_triple.carbon

@@ -3,7 +3,7 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 // ARGS: dump assembly --target_triple=x86_687-unknown-linux-gnu %s
-// TODO: Support autoupdate with ARGS.
+// No autoupdate because the message comes from LLVM.
 // NOAUTOUPDATE
 // CHECK:STDERR: ERROR: Invalid -target_triple:{{.*}}