Auto merge of #7039 - phansch:melt-ice, r=flip1995

tabs_in_doc_comments: Fix ICE due to char indexing

This is a quick-fix for an ICE in `tabs_in_doc_comments`. The problem
was that we we're indexing into possibly multi-byte characters, such as '位'.

More specifically `get_chunks_of_tabs` was returning indices into
multi-byte characters. Those were passed on to a `Span` creation that
then caused the ICE.

This fix makes sure that we don't return indices that point inside a
multi-byte character. *However*, we are still iterating over unicode
codepoints, not grapheme clusters. So a seemingly single character like y̆ ,
which actually consists of two codepoints, will probably still cause
incorrect spans in the output. But I don't think we handle those cases
anywhere in Clippy currently?

Fixes #5835

changelog: Fix ICE in `tabs_in_doc_comments`

tests/ui/crashes/ice-5835.rs

@@ -0,0 +1,9 @@
+#[rustfmt::skip]
+pub struct Foo {
+    /// 位	
+    ///   ^ Do not remove this tab character.
+    ///   It was required to trigger the ICE.
+    pub bar: u8,
+}
+
+fn main() {}

tests/ui/crashes/ice-5835.stderr

@@ -0,0 +1,10 @@
+error: using tabs in doc comments is not recommended
+  --> $DIR/ice-5835.rs:3:10
+   |
+LL |     /// 位    
+   |           ^^^^ help: consider using four spaces per tab
+   |
+   = note: `-D clippy::tabs-in-doc-comments` implied by `-D warnings`
+
+error: aborting due to previous error
+