introduce a unit testing feature rustc_evaluate_where_clauses

This attribute will cause us to invoke evaluate on every where clause of an
invoked function and to generate an error with the result.

Without this, it is very difficult to observe the effects of invoking the trait
evaluator.

compiler/rustc_span/src/symbol.rs

@@ -1010,6 +1010,7 @@ symbols! {
         rustc_dump_program_clauses,
         rustc_dump_user_substs,
         rustc_error,
+        rustc_evaluate_where_clauses,
         rustc_expected_cgu_reuse,
         rustc_if_this_changed,
         rustc_inherit_overflow_checks,