Use fewer origins when creating type variables.

`InferCtxt::next_{ty,const}_var*` all take an origin, but the
`param_def_id` is almost always `None`. This commit changes them to just
take a `Span` and build the origin within the method, and adds new
methods for the rare cases where `param_def_id` might not be `None`.
This avoids a lot of tedious origin building.

Specifically:
- next_ty_var{,_id_in_universe,_in_universe}: now take `Span` instead of
  `TypeVariableOrigin`
- next_ty_var_with_origin: added

- next_const_var{,_in_universe}: takes Span instead of ConstVariableOrigin
- next_const_var_with_origin: added

- next_region_var, next_region_var_in_universe: these are unchanged,
  still take RegionVariableOrigin

The API inconsistency (ty/const vs region) seems worth it for the
large conciseness improvements.

compiler/rustc_hir_analysis/src/check/compare_impl_item.rs

@@ -9,7 +9,6 @@ use rustc_hir::def::{DefKind, Res};
 use rustc_hir::intravisit;
 use rustc_hir::{GenericParamKind, ImplItemKind};
 use rustc_infer::infer::outlives::env::OutlivesEnvironment;
-use rustc_infer::infer::type_variable::TypeVariableOrigin;
 use rustc_infer::infer::{self, InferCtxt, TyCtxtInferExt};
 use rustc_infer::traits::{util, FulfillmentError};
 use rustc_middle::ty::error::{ExpectedFound, TypeError};
@@ -800,10 +799,7 @@ impl<'tcx> TypeFolder<TyCtxt<'tcx>> for ImplTraitInTraitCollector<'_, 'tcx> {
                 bug!("FIXME(RPITIT): error here");
             }
             // Replace with infer var
-            let infer_ty = self
-                .ocx
-                .infcx
-                .next_ty_var(TypeVariableOrigin { span: self.span, param_def_id: None });
+            let infer_ty = self.ocx.infcx.next_ty_var(self.span);
             self.types.insert(proj.def_id, (infer_ty, proj.args));
             // Recurse into bounds
             for (pred, pred_span) in self