use the closure def-id in returns, but closure-base def-id in locals

Using the `closure_base_def_id` indiscriminantely, as we were doing
before, winds up "going wrong" if the closure type includes the `impl
Trait` from the parent. The problem arises because the return value
for closures is inferred and meant to treat the return
type *opaquely*, so we don't want to be "desugaring" it into the
underlying type.

src/test/ui/impl-trait/closure-calling-parent-fn.rs

@@ -0,0 +1,14 @@
+#![feature(nll)]
+
+// Regression test for #54593: the MIR type checker was going wrong
+// when a closure returns the `impl Copy` from its parent fn. It was
+// (incorrectly) replacing the `impl Copy` in its return type with the
+// hidden type (`()`) but that type resulted from a recursive call to
+// `foo` and hence is treated opaquely within the closure body.  This
+// resulted in a failed subtype relationship.
+//
+// run-pass
+
+fn foo() -> impl Copy { || foo(); }
+fn bar() -> impl Copy { || bar(); }
+fn main() { }