Remove unreachable fatal error

src/base.rs

@@ -907,7 +907,7 @@ fn codegen_stmt<'tcx>(
         | StatementKind::PlaceMention(..)
         | StatementKind::AscribeUserType(..) => {}
 
-        StatementKind::Coverage { .. } => fx.tcx.dcx().fatal("-Zcoverage is unimplemented"),
+        StatementKind::Coverage { .. } => unreachable!(),
         StatementKind::Intrinsic(ref intrinsic) => match &**intrinsic {
             // We ignore `assume` intrinsics, they are only useful for optimizations
             NonDivergingIntrinsic::Assume(_) => {}