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## Summary

<!-- What's the purpose of the change? What does it do, and why? -->

From @ntBre
#18906 (comment) :
> This could be a good target for a follow-up PR, but we could fold
these `if checker.is_rule_enabled { checker.report_diagnostic` checks
into calls to `checker.report_diagnostic_if_enabled`. I didn't notice
these when adding that method.
> 
> Also, the docs on `Checker::report_diagnostic_if_enabled` and
`LintContext::report_diagnostic_if_enabled` are outdated now that the
`Rule` conversion is basically free 😅
> 
> No pressure to take on this refactor, just an idea if you're
interested!

This PR folds those calls. I also updated the doc comments by copying
from `report_diagnostic`.

Note: It seems odd to me that the doc comment for `Checker` says
`Diagnostic` while `LintContext` says `OldDiagnostic`, not sure if that
needs a bigger docs change to fix the inconsistency.

<details>
<summary>Python script to do the changes</summary>

This script assumes it is placed in the top level `ruff` directory (ie
next to `.git`/`crates`/`README.md`)

```py
import re
from copy import copy
from pathlib import Path

ruff_crates = Path(__file__).parent / "crates"

for path in ruff_crates.rglob("**/*.rs"):
    with path.open(encoding="utf-8", newline="") as f:
        original_content = f.read()
    if "is_rule_enabled" not in original_content or "report_diagnostic" not in original_content:
        continue
    original_content_position = 0
    changed_content = ""
    for match in re.finditer(r"(?m)(?:^[ \n]*|(?<=(?P<else>else )))if[ \n]+checker[ \n]*\.is_rule_enabled\([ \n]*Rule::\w+[ \n]*\)[ \n]*{[ \n]*checker\.report_diagnostic\(", original_content):
        # Content between last match and start of this one is unchanged
        changed_content += original_content[original_content_position:match.start()]
        # If this was an else if, a { needs to be added at the start
        if match.group("else"):
            changed_content += "{"
        # This will result in bad formatting, but the precommit cargo format will handle it
        changed_content += "checker.report_diagnostic_if_enabled("
        # Depth tracking would fail if a string/comment included a { or }, but unlikely given the context
        depth = 1
        position = match.end()
        while depth > 0:
            if original_content[position] == "{":
                depth += 1
            if original_content[position] == "}":
                depth -= 1
            position += 1
        # pos - 1 is the closing }
        changed_content += original_content[match.end():position - 1]
        # If this was an else if, a } needs to be added at the end
        if match.group("else"):
            changed_content += "}"
        # Skip the closing }
        original_content_position = position
        if original_content[original_content_position] == "\n":
            # If the } is followed by a \n, also skip it for better formatting
            original_content_position += 1
    # Add remaining content between last match and file end
    changed_content += original_content[original_content_position:]
    with path.open("w", encoding="utf-8", newline="") as f:
        f.write(changed_content)
```

</details>

## Test Plan

<!-- How was it tested? -->

N/A, no tests/functionality affected.

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## Summary

<!-- What's the purpose of the change? What does it do, and why? -->

Here's the part that was split out of #18906. I wanted to move these
into the rule files since the rest of the rules in
`deferred_scope`/`statement` have that same structure of implementations
being in the rule definition file. It also resolves the dilemma of where
to put the comment, at least for these rules.

## Test Plan

<!-- How was it tested? -->

N/A, no test/functionality affected