Move `scripts` on the rustdoc template into `head` and apply the `defer` attribute

Closes https://github.com/rust-lang/rust/issues/90719

Check for duplicate attributes.

This adds some checks for duplicate attributes. In many cases, the duplicates were being ignored without error or warning. This adds several kinds of checks (see `AttributeDuplicates` enum).

The motivation here is to issue unused warnings with similar reasoning for any unused lint, and to error for cases where there are conflicts.

This also adds a check for empty attribute lists in a few attributes where this causes the attribute to be ignored.

Closes #55112.

Simplify `for` loop desugar

Basically two intermediate bindings are inlined. I could have left one intermediate binding in place as this would simplify some diagnostic logic, but I think the difference in that regard would be negligible, so it is better to have a minimal HIR.

For checking that the pattern is irrefutable, I added a special case when the `match` is found to be non-exhaustive.

The reordering of the arms is purely stylistic. I don't *think* there are any perf implications.

```diff
  match IntoIterator::into_iter($head) {
      mut iter => {
          $label: loop {
-             let mut __next;
              match Iterator::next(&mut iter) {
-                 Some(val) => __next = val,
                  None => break,
+                 Some($pat) => $block,
              }
-             let $pat = __next;
-             $block
          }
      }
  }
```

libcore: assume the input of `next_code_point` and `next_code_point_reverse` is UTF-8-like

The functions are now `unsafe` and they use `Option::unwrap_unchecked` instead of `unwrap_or_0`

`unwrap_or_0` was added in 42357d772b8a3a1ce4395deeac0a5cf1f66e951d. I guess `unwrap_unchecked` was not available back then.

Given this example:

```rust
pub fn first_char(s: &str) -> Option<char> {
    s.chars().next()
}
```

Previously, the following assembly was produced:

```asm
_ZN7example10first_char17ha056ddea6bafad1cE:
	.cfi_startproc
	test	rsi, rsi
	je	.LBB0_1
	movzx	edx, byte ptr [rdi]
	test	dl, dl
	js	.LBB0_3
	mov	eax, edx
	ret
.LBB0_1:
	mov	eax, 1114112
	ret
.LBB0_3:
	lea	r8, [rdi + rsi]
	xor	eax, eax
	mov	r9, r8
	cmp	rsi, 1
	je	.LBB0_5
	movzx	eax, byte ptr [rdi + 1]
	add	rdi, 2
	and	eax, 63
	mov	r9, rdi
.LBB0_5:
	mov	ecx, edx
	and	ecx, 31
	cmp	dl, -33
	jbe	.LBB0_6
	cmp	r9, r8
	je	.LBB0_9
	movzx	esi, byte ptr [r9]
	add	r9, 1
	and	esi, 63
	shl	eax, 6
	or	eax, esi
	cmp	dl, -16
	jb	.LBB0_12
.LBB0_13:
	cmp	r9, r8
	je	.LBB0_14
	movzx	edx, byte ptr [r9]
	and	edx, 63
	jmp	.LBB0_16
.LBB0_6:
	shl	ecx, 6
	or	eax, ecx
	ret
.LBB0_9:
	xor	esi, esi
	mov	r9, r8
	shl	eax, 6
	or	eax, esi
	cmp	dl, -16
	jae	.LBB0_13
.LBB0_12:
	shl	ecx, 12
	or	eax, ecx
	ret
.LBB0_14:
	xor	edx, edx
.LBB0_16:
	and	ecx, 7
	shl	ecx, 18
	shl	eax, 6
	or	eax, ecx
	or	eax, edx
	ret
```

After this change, the assembly is reduced to:

```asm
_ZN7example10first_char17h4318683472f884ccE:
	.cfi_startproc
	test	rsi, rsi
	je	.LBB0_1
	movzx	ecx, byte ptr [rdi]
	test	cl, cl
	js	.LBB0_3
	mov	eax, ecx
	ret
.LBB0_1:
	mov	eax, 1114112
	ret
.LBB0_3:
	mov	eax, ecx
	and	eax, 31
	movzx	esi, byte ptr [rdi + 1]
	and	esi, 63
	cmp	cl, -33
	jbe	.LBB0_4
	movzx	edx, byte ptr [rdi + 2]
	shl	esi, 6
	and	edx, 63
	or	edx, esi
	cmp	cl, -16
	jb	.LBB0_7
	movzx	ecx, byte ptr [rdi + 3]
	and	eax, 7
	shl	eax, 18
	shl	edx, 6
	and	ecx, 63
	or	ecx, edx
	or	eax, ecx
	ret
.LBB0_4:
	shl	eax, 6
	or	eax, esi
	ret
.LBB0_7:
	shl	eax, 12
	or	eax, edx
	ret
```

The functions are now `unsafe` and they use `Option::unwrap_unchecked` instead of `unwrap_or_0`

`unwrap_or_0` was added in 42357d772b8a3a1ce4395deeac0a5cf1f66e951d. I guess `unwrap_unchecked` was not available back then.

Given this example:

```rust
pub fn first_char(s: &str) -> Option<char> {
    s.chars().next()
}
```

Previously, the following assembly was produced:

```asm
_ZN7example10first_char17ha056ddea6bafad1cE:
	.cfi_startproc
	test	rsi, rsi
	je	.LBB0_1
	movzx	edx, byte ptr [rdi]
	test	dl, dl
	js	.LBB0_3
	mov	eax, edx
	ret
.LBB0_1:
	mov	eax, 1114112
	ret
.LBB0_3:
	lea	r8, [rdi + rsi]
	xor	eax, eax
	mov	r9, r8
	cmp	rsi, 1
	je	.LBB0_5
	movzx	eax, byte ptr [rdi + 1]
	add	rdi, 2
	and	eax, 63
	mov	r9, rdi
.LBB0_5:
	mov	ecx, edx
	and	ecx, 31
	cmp	dl, -33
	jbe	.LBB0_6
	cmp	r9, r8
	je	.LBB0_9
	movzx	esi, byte ptr [r9]
	add	r9, 1
	and	esi, 63
	shl	eax, 6
	or	eax, esi
	cmp	dl, -16
	jb	.LBB0_12
.LBB0_13:
	cmp	r9, r8
	je	.LBB0_14
	movzx	edx, byte ptr [r9]
	and	edx, 63
	jmp	.LBB0_16
.LBB0_6:
	shl	ecx, 6
	or	eax, ecx
	ret
.LBB0_9:
	xor	esi, esi
	mov	r9, r8
	shl	eax, 6
	or	eax, esi
	cmp	dl, -16
	jae	.LBB0_13
.LBB0_12:
	shl	ecx, 12
	or	eax, ecx
	ret
.LBB0_14:
	xor	edx, edx
.LBB0_16:
	and	ecx, 7
	shl	ecx, 18
	shl	eax, 6
	or	eax, ecx
	or	eax, edx
	ret
```

After this change, the assembly is reduced to:

```asm
_ZN7example10first_char17h4318683472f884ccE:
	.cfi_startproc
	test	rsi, rsi
	je	.LBB0_1
	movzx	ecx, byte ptr [rdi]
	test	cl, cl
	js	.LBB0_3
	mov	eax, ecx
	ret
.LBB0_1:
	mov	eax, 1114112
	ret
.LBB0_3:
	mov	eax, ecx
	and	eax, 31
	movzx	esi, byte ptr [rdi + 1]
	and	esi, 63
	cmp	cl, -33
	jbe	.LBB0_4
	movzx	edx, byte ptr [rdi + 2]
	shl	esi, 6
	and	edx, 63
	or	edx, esi
	cmp	cl, -16
	jb	.LBB0_7
	movzx	ecx, byte ptr [rdi + 3]
	and	eax, 7
	shl	eax, 18
	shl	edx, 6
	and	ecx, 63
	or	ecx, edx
	or	eax, ecx
	ret
.LBB0_4:
	shl	eax, 6
	or	eax, esi
	ret
.LBB0_7:
	shl	eax, 12
	or	eax, edx
	ret
```

This removes the duplicate check, as this is now handled in a
centralized location.