[CAS] OnDiskGraphDB - fix MSVC "not all control paths return a value" warnings. NFC. (#164369)

Author: RKSimon

llvm/include/llvm/CAS/OnDiskGraphDB.h

@@ -380,6 +380,7 @@ class OnDiskGraphDB {
     case ObjectPresence::OnlyInUpstreamDB:
       return true;
     }
+    llvm_unreachable("Unknown ObjectPresence enum");
   }
 
   /// When \p load is called for a node that doesn't exist, this function tries

llvm/lib/CAS/OnDiskGraphDB.cpp

@@ -836,6 +836,7 @@ uint64_t DataRecordHandle::getDataSize() const {
   case DataSizeFlags::Uses8B:
     return support::endian::read64le(DataSizePtr);
   }
+  llvm_unreachable("Unknown DataSizeFlags enum");
 }
 
 void DataRecordHandle::skipDataSize(LayoutFlags LF, int64_t &RelOffset) const {
@@ -863,6 +864,7 @@ uint32_t DataRecordHandle::getNumRefs() const {
   case NumRefsFlags::Uses8B:
     return support::endian::read64le(NumRefsPtr);
   }
+  llvm_unreachable("Unknown NumRefsFlags enum");
 }
 
 void DataRecordHandle::skipNumRefs(LayoutFlags LF, int64_t &RelOffset) const {
@@ -1270,6 +1272,7 @@ Expected<bool> OnDiskGraphDB::isMaterialized(ObjectID Ref) {
       return FaultInResult.takeError();
     return true;
   }
+  llvm_unreachable("Unknown ObjectPresence enum");
 }
 
 Expected<OnDiskGraphDB::ObjectPresence>