Make MCDCRecord::getNumConditions() const&

Some users were trying to get a reference to the return value.

Author: chapuni

llvm/include/llvm/ProfileData/Coverage/CoverageMapping.h

@@ -461,7 +461,7 @@ struct MCDCRecord {
         Folded(std::move(Folded)), PosToID(std::move(PosToID)),
         CondLoc(std::move(CondLoc)){};
 
-  CounterMappingRegion getDecisionRegion() const { return Region; }
+  const CounterMappingRegion &getDecisionRegion() const { return Region; }
   unsigned getNumConditions() const {
     return Region.getDecisionParams().NumConditions;
   }