Cave in and just add the unreachable's gcc wants

ConflictSet.cpp

@@ -474,6 +474,7 @@ Node *&getChildExists(Node *self, uint8_t index) {
     return self256->children[index].child;
   }
   }
+  __builtin_unreachable(); // GCOVR_EXCL_LINE
 }
 
 // Precondition - an entry for index must exist in the node
@@ -505,6 +506,7 @@ Node *getChild(Node *self, uint8_t index) {
     return self256->children[index].child;
   }
   }
+  __builtin_unreachable(); // GCOVR_EXCL_LINE
 }
 
 template <class NodeT> int getChildGeqSimd(NodeT *self, int child) {
@@ -584,6 +586,7 @@ int getChildGeq(Node *self, int child) {
     return self48->bitSet.firstSetGeq(child);
   }
   }
+  __builtin_unreachable(); // GCOVR_EXCL_LINE
 }
 
 void setChildrenParents(Node3 *n) {
@@ -776,6 +779,7 @@ Node *&getOrCreateChild(Node *&self, uint8_t index,
     return self256->children[index].child;
   }
   }
+  __builtin_unreachable(); // GCOVR_EXCL_LINE
 }
 
 Node *nextPhysical(Node *node) {
@@ -2344,6 +2348,7 @@ int64_t &maxVersion(Node *n, ConflictSet::Impl *impl) {
     return n256->children[index].childMaxVersion;
   }
   }
+  __builtin_unreachable(); // GCOVR_EXCL_LINE
 }
 
 Node *&getInTree(Node *n, ConflictSet::Impl *impl) {