rustdoc: skip allow missing doc in cover. report

During the document coverage reporting with
```bash
rustdoc something.rs -Z unstable-options --show-coverage
```

the coverage report also includes parts of the code that are marked
with `#[allow(missing_docs)]`, which outputs lower numbers in the
coverage report even though these parts should be ignored for the
calculation.

Co-authored-by: Joshua Nelson <joshua@yottadb.com>

src/test/rustdoc-ui/coverage/allow_missing_docs.rs

@@ -0,0 +1,31 @@
+// compile-flags:-Z unstable-options --show-coverage
+// check-pass
+
+//! Make sure to have some docs on your crate root
+
+#[allow(missing_docs)]
+pub mod mod_foo {
+    pub struct Bar;
+}
+
+/// This is a struct with a `#[allow(missing_docs)]`
+pub struct AllowTheMissingDocs {
+    #[allow(missing_docs)]
+    pub empty_str: String,
+
+    /// This has
+    #[allow(missing_docs)]
+    /// but also has documentation comments
+    pub hello: usize,
+
+    /// The doc id just to create a boilerplate comment
+    pub doc_id: Vec<u8>,
+}
+
+/// A function that has a documentation
+pub fn this_is_func() {}
+
+#[allow(missing_docs)]
+pub struct DemoStruct {
+    something: usize,
+}