Rename GenericArg::id as GenericArg::hir_id.

Because `hir_id` is the standard name for methods that return a `HirId`
from a HIR node.

compiler/rustc_hir/src/hir.rs

@@ -280,7 +280,7 @@ impl GenericArg<'_> {
         }
     }
 
-    pub fn id(&self) -> HirId {
+    pub fn hir_id(&self) -> HirId {
         match self {
             GenericArg::Lifetime(l) => l.hir_id,
             GenericArg::Type(t) => t.hir_id,