Rollup merge of #94137 - aDotInTheVoid:abi-enum, r=CraftSpider

rustdoc-json: Better Header Type

- Make ABI an enum, instead of being stringly typed
- Replace Qualifier HashSet with 3 bools
- Merge ABI field into header, as they always occor together

r? ``@CraftSpider``

``@rustbot`` modify labels: +A-rustdoc-json +T-rustdoc

compiler/rustc_hir/src/hir.rs

@@ -2776,6 +2776,10 @@ impl FnHeader {
     pub fn is_const(&self) -> bool {
         matches!(&self.constness, Constness::Const)
     }
+
+    pub fn is_unsafe(&self) -> bool {
+        matches!(&self.unsafety, Unsafety::Unsafe)
+    }
 }
 
 #[derive(Debug, HashStable_Generic)]