Exhaustively specify names and stability of --print values

[Zalathar]

While trying to add a new unstable --print kind for use by compiletest, I found that the relevant code is quite awkward to work with, for a few reasons:

• It's spread across various parts of a multi-thousand-line source file.
• All newly-added PrintKind values are automatically treated as stable, unless they are explicitly marked as unstable in a helper function far away.
• Parsing --print values relies on a separate table of name/value mappings, but there's no exhaustiveness check for that table.

This PR therefore:

• Extracts the relevant code into its own print_request submodule.
• Uses a macro-rules derive to obtain an exhaustive list of values.
• Uses exhaustive matches to associate a name and stability status with each PrintKind value.

---

The first commit moves code to a separate module; the second commit contains actual changes.

There should be no change to compiler output.

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[Zalathar]

Because r-a doesn't support macro-rules derives yet (rust-lang/rust-analyzer#21043), autocomplete fails for any items introduced by the derive. So I've been reluctant to macro-rules derives more widely within the compiler.

But in this case there's a relatively simple workaround, which is to alias the trait associated constant to a regular associated constant.