Create stable metric to measure long computation in Const Eval

This patch adds a `MirPass` that tracks the number of back-edges and
function calls in the CFG, adds a new MIR instruction to increment a
counter every time they are encountered during Const Eval, and emit a
warning if a configured limit is breached.

compiler/rustc_mir_transform/src/generator.rs

@@ -1583,6 +1583,7 @@ impl<'tcx> Visitor<'tcx> for EnsureGeneratorFieldAssignmentsNeverAlias<'_> {
             | StatementKind::AscribeUserType(..)
             | StatementKind::Coverage(..)
             | StatementKind::Intrinsic(..)
+            | StatementKind::ConstEvalCounter
             | StatementKind::Nop => {}
         }
     }