Merge toolchain file_test children in order to improve linking. (#3206)

Specifically this should improve linking by producing one large binary
instead of one-per-directory. The inclusion of the driver hits the size
issue. Separating out things which have more llvm deps has been
discussed, but I'm not doing that here because I think the semantics
layer will need to depend on clang for interop, and we'd lose a lot of
the benefits that way. Also, having just one place to look seems
simpler.

Includes supporting changes to file_test infrastructure, the most
significant of which is probably passing tests via file instead of a
really large args thing, using a custom rule to do that. That's because
dealing with the layered filegroups that allow the toolchain setup is
more complicated, and this approach scales well.

Combined test time is ~9s, so not sharding right now.

I wasn't sure if people would prefer having the autoupdate script under
testing, so I left it alone for now.

testing/file_test/file_test_base_test.cpp

@@ -31,7 +31,7 @@ class FileTestBaseTest : public FileTestBase {
     }
     stdout << "\n";
 
-    auto filename = path().filename();
+    auto filename = std::filesystem::path(test_name().str()).filename();
     if (filename == "args.carbon") {
       // 'args.carbon' has custom arguments, so don't do regular argument
       // validation for it.