Fix MSVC "not all control paths return a value" warning. NFCI.

llvm-svn: 370343

clang-tools-extra/clangd/refactor/tweaks/ExtractFunction.cpp

@@ -132,6 +132,7 @@ const Node *getParentOfRootStmts(const Node *CommonAnc) {
     // always unselected.
     return Parent->ASTNode.get<DeclStmt>() ? Parent->Parent : Parent;
   }
+  llvm_unreachable("Unhandled SelectionTree::Selection enum");
 }
 
 // The ExtractionZone class forms a view of the code wrt Zone.