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_borrowck/src/lib.rs

@@ -620,7 +620,8 @@ impl<'cx, 'tcx> rustc_mir_dataflow::ResultsVisitor<'cx, 'tcx> for MirBorrowckCtx
                     flow_state,
                 );
             }
-            StatementKind::Nop
+            StatementKind::ConstEvalCounter
+            | StatementKind::Nop
             | StatementKind::Retag { .. }
             | StatementKind::Deinit(..)
             | StatementKind::SetDiscriminant { .. } => {