Rollup merge of #128045 - pnkfelix:rustc-contracts, r=oli-obk

#[contracts::requires(...)]  + #[contracts::ensures(...)]

cc https://github.com/rust-lang/rust/issues/128044

Updated contract support: attribute syntax for preconditions and postconditions, implemented via a series of desugarings  that culminates in:
1. a compile-time flag (`-Z contract-checks`) that, similar to `-Z ub-checks`, attempts to ensure that the decision of enabling/disabling contract checks is delayed until the end user program is compiled,
2. invocations of lang-items that handle invoking the precondition,  building a checker for the post-condition, and invoking that post-condition checker at the return sites for the function, and
3. intrinsics for the actual evaluation of pre- and post-condition predicates that third-party verification tools can intercept and reinterpret for their own purposes (e.g. creating shims of behavior that abstract away the function body and replace it solely with the pre- and post-conditions).

Known issues:

 * My original intent, as described in the MCP (https://github.com/rust-lang/compiler-team/issues/759) was   to have a rustc-prefixed attribute namespace (like   rustc_contracts::requires). But I could not get things working when I tried   to do rewriting via a rustc-prefixed builtin attribute-macro. So for now it  is called `contracts::requires`.

 * Our attribute macro machinery does not provide direct support for attribute arguments that are parsed like rust expressions. I spent some time trying to add that (e.g. something that would parse the attribute arguments as an AST while treating the remainder of the items as a token-tree), but its too big a lift for me to undertake. So instead I hacked in something approximating that goal, by semi-trivially desugaring the token-tree attribute contents into internal AST constucts. This may be too fragile for the long-term.
   * (In particular, it *definitely* breaks when you try to add a contract to a function like this: `fn foo1(x: i32) -> S<{ 23 }> { ... }`, because its token-tree based search for where to inject the internal AST constructs cannot immediately see that the `{ 23 }` is within a generics list. I think we can live for this for the short-term, i.e. land the work, and continue working on it while in parallel adding a new attribute variant that takes a token-tree attribute alongside an AST annotation, which would completely resolve the issue here.)

* the *intent* of `-Z contract-checks` is that it behaves like `-Z ub-checks`, in that we do not prematurely commit to including or excluding the contract evaluation in upstream crates (most notably, `core` and `std`). But the current test suite does not actually *check* that this is the case. Ideally the test suite would be extended with a multi-crate test that explores the matrix of enabling/disabling contracts on both the upstream lib and final ("leaf") bin crates.

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