Start including the prelude for testing. (#3861)

- Adds an empty prelude.carbon file
- Imports that file in any non-Core package file
  - Adds --disable-prelude-import to avoid that
- Adds --exclude-dump-file-prefix to be able to hide files from dumping
- Used to hide core files (we can't do this by package name due to lex
dumps, for example)
- Restructures some tests to not rely on `i32`, particularly `alias`
tests (which rely on a name ref) and tests with no prelude.

I'm adding the framework for switching i32 to calling Int32 in the
prelude, but I'm running into a separate error actually switching over.
So that *mostly* works, but isn't quite ready for prime time. However,
maybe the current state of this PR is still useful to review since it
does a lot of the infrastructure work and adds the %Core everywhere?

toolchain/codegen/testdata/objcode/basic.carbon

@@ -2,9 +2,9 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// ARGS: compile --target=x86_64-unknown-linux-gnu --output=%t %s
+// ARGS: compile --no-prelude-import --target=x86_64-unknown-linux-gnu --output=%t %s
 //
 // TODO: Add a way to write some basic tests for object file outputs.
 // AUTOUPDATE
 
-fn Main() -> i32 { return 0; }
+fn Main() {}