Use `check_proc_macro` for `missing_const_for_fn`

This uses `@Jarcho's` #8694 implementation to fix `missing_const_for_fn` linting in proc-macros.
I'm not 100% sure what I'm doing here, any feedback is appreciated.

Previously: Jarcho#1.
Fixes #8854.

changelog: [`missing_const_for_fn`]: No longer lints in proc-macros

…=xFrednet

Fix tuple_array_conversions lint on nightly

```
changelog: ICE: [`tuple_array_conversions`]: Don't expect array length to always be usize
```

tl;dr: changed [`Const::eval_target_usize`](https://github.com/rust-lang/rust/blob/master/compiler/rustc_middle/src/ty/consts.rs#L359) to [`Consts::try_eval_target_usize`](https://github.com/rust-lang/rust/blob/master/compiler/rustc_middle/src/ty/consts.rs#L327) to get rid of ICE.

I have encountered a problem with clippy: it caught ICE when working with a codebase that uses a lot of nightly features.
Here's a (stripped) ICE info:

```
error: internal compiler error: /rustc/5c6a7e71cd66705c31c9af94077901a220f0870c/compiler/rustc_middle/src/ty/consts.rs:361:32: expected usize, got Const { ty: usize, kind: N/#1 }

thread 'rustc' panicked at /rustc/5c6a7e71cd66705c31c9af94077901a220f0870c/compiler/rustc_errors/src/lib.rs:1635:9:
Box<dyn Any>
stack backtrace:
...
  16:        0x110b9c590 - rustc_middle[449edf845976488d]::util::bug::bug_fmt
  17:        0x102f76ae0 - clippy_lints[71754038dd04c2d2]::tuple_array_conversions::all_bindings_are_for_conv
...
```

I don't really know what's going on low-level-wise, but seems like this lin assumed that the length of the array can always be treated as `usize`, and *I assume* this doesn't play well with `feat(generic_const_exprs)`.

I wasn't able to build a minimal reproducible example, but locally this fix does resolve the issue.

fix: check against 'main' function name instead of entrypoint function

This resulted in the following error for me:

    error[E0464]: multiple candidates for `rlib` dependency `rustc_literal_escaper` found
      --> clippy_dev/src/lib.rs:25:1
       |
    25 | extern crate rustc_literal_escaper;
       | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
       |
       = note: candidate #1: /home/pkrones/.rustup/toolchains/nightly-2025-12-25-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/librustc_literal_escaper-4f30574e01c3dad1.rlib
       = note: candidate rust-lang#2: /home/pkrones/.rustup/toolchains/nightly-2025-12-25-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/librustc_literal_escaper-c8a0e29ad1040cee.rmeta

Other tools in the Rust repo also take that as an explicit dependency, like rust-analyzer or lint-docs.