Fix RegionCtxt::preference_value.

There's a bad pattern matching confusion present in this function.
`_anon` gets assigned to, and then `_anon` is used as an unbound
variable in the pattern, which is unrelated to the first `_anon`.
If the `_anon` didn't start with `_` the compiler would give warnings.

This was introduced in #104239.

I have rewritten the function to remove the confusion and preserve the
existing behaviour. This seems safest, because the original intent is
not clear.

compiler/rustc_borrowck/src/renumber.rs

@@ -69,12 +69,10 @@ impl RegionCtxt {
     /// Used to determine the representative of a component in the strongly connected
     /// constraint graph
     pub(crate) fn preference_value(self) -> usize {
-        let _anon = Symbol::intern("anon");
-
         match self {
             RegionCtxt::Unknown => 1,
             RegionCtxt::Existential(None) => 2,
-            RegionCtxt::Existential(Some(_anon)) | RegionCtxt::Free(_anon) => 2,
+            RegionCtxt::Existential(Some(_)) | RegionCtxt::Free(_) => 2,
             RegionCtxt::Location(_) => 3,
             RegionCtxt::TyContext(_) => 4,
             _ => 5,