Rollup merge of rust-lang#119532 - GKFX:offset-of-parse-expr, r=est31

Make offset_of field parsing use metavariable which handles any spacing

As discussed at and around comments rust-lang#106655 (comment) and rust-lang#106655 (comment), the current arguments to offset_of do not accept all the whitespace combinations: `0. 1.1.1` and `0.1.1. 1` are currently treated specially in `tests/ui/offset-of/offset-of-tuple-nested.rs`.

They also do not allow [forwarding individual fields as in](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=444cdf0ec02b99e8fd5fd8d8ecb312ca)
```rust
macro_rules! off {
    ($a:expr) => {
        offset_of!(m::S, 0. $a)
    }
}
```

This PR replaces the macro arguments with `($Container:ty, $($fields:expr)+ $(,)?)` which does allow any arrangement of whitespace that I could come up with and the forwarding of fields example above.

This also allows for array indexing in the future, which I think is the last future extension to the syntax suggested in the offset_of RFC.

Tracking issue for offset_of: rust-lang#106655
``@rustbot`` label F-offset_of

``@est31``

Author: matthiaskrgr

core/src/mem/mod.rs

@@ -1395,8 +1395,18 @@ impl<T> SizedTypeProperties for T {}
 ///
 /// assert_eq!(mem::offset_of!(Option<&u8>, Some.0), 0);
 /// ```
+#[cfg(not(bootstrap))]
 #[unstable(feature = "offset_of", issue = "106655")]
 #[allow_internal_unstable(builtin_syntax, hint_must_use)]
+pub macro offset_of($Container:ty, $($fields:expr)+ $(,)?) {
+    // The `{}` is for better error messages
+    crate::hint::must_use({builtin # offset_of($Container, $($fields)+)})
+}
+
+#[cfg(bootstrap)]
+#[unstable(feature = "offset_of", issue = "106655")]
+#[allow_internal_unstable(builtin_syntax, hint_must_use)]
+#[allow(missing_docs)]
 pub macro offset_of($Container:ty, $($fields:tt).+ $(,)?) {
     // The `{}` is for better error messages
     crate::hint::must_use({builtin # offset_of($Container, $($fields).+)})