Rollup merge of #92875 - BoxyUwU:infer_arg_opt_const_param_of, r=lcnr

Make `opt_const_param_of` work in the presence of `GenericArg::Infer`

highly recommend viewing the first and second commits on their own rather than looking at file changes 🤣

Because we filtered args down to just const args we would ignore `GenericArg::Infer` which made us get a `arg_index` which was wrong by however many const `GenericArg::Infer` came previously

[example](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=46dba6a53aca6333028a10908ef16e0b) of the [bugs](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=a8eebced26eefa4119fc2e7ae0c76de6) fixed.

r? ```@lcnr```

Author: matthiaskrgr

compiler/rustc_hir/src/hir.rs

@@ -293,10 +293,6 @@ impl GenericArg<'_> {
         }
     }
 
-    pub fn is_const(&self) -> bool {
-        matches!(self, GenericArg::Const(_))
-    }
-
     pub fn is_synthetic(&self) -> bool {
         matches!(self, GenericArg::Lifetime(lifetime) if lifetime.name.ident() == Ident::empty())
     }
@@ -318,6 +314,13 @@ impl GenericArg<'_> {
             GenericArg::Infer(_) => ast::ParamKindOrd::Infer,
         }
     }
+
+    pub fn is_ty_or_const(&self) -> bool {
+        match self {
+            GenericArg::Lifetime(_) => false,
+            GenericArg::Type(_) | GenericArg::Const(_) | GenericArg::Infer(_) => true,
+        }
+    }
 }
 
 #[derive(Debug, HashStable_Generic)]

compiler/rustc_middle/src/ty/generics.rs

@@ -31,6 +31,13 @@ impl GenericParamDefKind {
             GenericParamDefKind::Const { .. } => ast::ParamKindOrd::Const,
         }
     }
+
+    pub fn is_ty_or_const(&self) -> bool {
+        match self {
+            GenericParamDefKind::Lifetime => false,
+            GenericParamDefKind::Type { .. } | GenericParamDefKind::Const { .. } => true,
+        }
+    }
 }
 
 #[derive(Clone, Debug, TyEncodable, TyDecodable, HashStable)]