[flang][OpenMP] fix diagnostic for bad cancel type (#140798)

Fixes #133685

Author: tblah

flang/lib/Semantics/check-omp-structure.cpp

@@ -2575,8 +2575,8 @@ void OmpStructureChecker::CheckCancellationNest(
       }
       break;
     default:
-      // This should have been diagnosed by this point.
-      llvm_unreachable("Unexpected directive");
+      // This is diagnosed later.
+      return;
     }
     if (!eligibleCancellation) {
       context_.Say(source,
@@ -2614,8 +2614,8 @@ void OmpStructureChecker::CheckCancellationNest(
           parser::ToUpperCaseLetters(typeName.str()));
       break;
     default:
-      // This should have been diagnosed by this point.
-      llvm_unreachable("Unexpected directive");
+      // This is diagnosed later.
+      return;
     }
   }
 }

flang/test/Semantics/OpenMP/cancel-bad-cancel-type.f90

@@ -0,0 +1,6 @@
+!RUN: %python %S/../test_errors.py %s %flang_fc1 %openmp_flags
+
+program test
+!ERROR: PARALLEL DO is not a cancellable construct
+!$omp cancel parallel do
+end