Rollup merge of #108726 - est31:backticks_matchmaking_tidy, r=Nilstrieb

tidy: enforce comment blocks to have an even number of backticks

After PR #108694, most unmatched backticks in `compiler/` comments have been eliminated. This PR adds a tidy lint to ensure no new unmatched backticks are added, and either addresses the lint in the remaining instances it found, or allows it.

Very often, backtick containing sections wrap around lines, for example:

```Rust
// This function takes a tuple `(Vec<String>,
// Box<[u8]>)` and transforms it into `Vec<u8>`.
```

The lint is implemented to work on top of blocks, counting each line with a `//` into a block, and counting if there are an odd or even number of backticks in the entire block, instead of looking at just a single line.

compiler/rustc_codegen_llvm/src/debuginfo/metadata/enums/native.rs

@@ -438,6 +438,7 @@ fn build_enum_variant_member_di_node<'ll, 'tcx>(
 ///         DW_TAG_structure_type            (type of variant 1)
 ///         DW_TAG_structure_type            (type of variant 2)
 ///         DW_TAG_structure_type            (type of variant 3)
+/// ```
 struct VariantMemberInfo<'a, 'll> {
     variant_index: VariantIdx,
     variant_name: Cow<'a, str>,