rustdoc: Properly inline const items

Build `clean::ConstantItem` values in the `inline` module and
pretty-print the AST for inlined const items.

Doc strings are still missing from inlined constants (see #19773).

Partially address #18156, #19722, #19185

Fix #15821

src/librustdoc/html/item_type.rs

@@ -76,6 +76,7 @@ impl ItemType {
             clean::TypeTrait    => ItemType::Trait,
             clean::TypeModule   => ItemType::Module,
             clean::TypeStatic   => ItemType::Static,
+            clean::TypeConst    => ItemType::Constant,
             clean::TypeVariant  => ItemType::Variant,
             clean::TypeTypedef  => ItemType::Typedef,
         }