Rollup merge of #120181 - dtolnay:tlconst, r=thomcc

Allow any `const` expression blocks in `thread_local!`

This PR contains a rebase of the macro change from #116392, together with adding a test under library/std/tests.

Testing this feature by making the documentation's example code needlessly more complicated was not appropriate as pointed out in https://github.com/rust-lang/rust/pull/116392#pullrequestreview-1753097757.

Without the macro change, this new test would fail to build as follows:

```console
error: no rules expected the token `let`
   --> library/std/tests/thread.rs:26:13
    |
26  |             let value = 1;
    |             ^^^ no rules expected this token in macro call
    |
note: while trying to match meta-variable `$init:expr`
   --> library/std/src/thread/local.rs:189:69
    |
189 |     ($(#[$attr:meta])* $vis:vis static $name:ident: $t:ty = const { $init:expr }; $($rest:tt)*) => (
    |                                                                     ^^^^^^^^^^
```

Closes #116392.

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