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/dataflow.rs

@@ -393,6 +393,7 @@ impl<'tcx> rustc_mir_dataflow::GenKillAnalysis<'tcx> for Borrows<'_, 'tcx> {
             | mir::StatementKind::AscribeUserType(..)
             | mir::StatementKind::Coverage(..)
             | mir::StatementKind::Intrinsic(..)
+            | mir::StatementKind::ConstEvalCounter
             | mir::StatementKind::Nop => {}
         }
     }