Fix language-server crash with cpp_ast (#5604)

Removes the nullptr default for safety.

Note, I think cpp support doesn't allow things like `<version>` or
inline code yet, and language-server support doesn't allow non-hermetic
files, so the best I can test is an error.

toolchain/language_server/context.cpp

@@ -140,14 +140,17 @@ auto Context::File::SetText(Context& context, std::optional<int64_t> version,
 
   SemIR::File sem_ir(tree_.get(), SemIR::CheckIRId(0), tree_->packaging_decl(),
                      *value_stores_, uri_.file().str());
-  auto getter = [this]() -> const Parse::TreeAndSubtrees& {
-    return *tree_and_subtrees_;
-  };
+  std::unique_ptr<clang::ASTUnit> cpp_ast;
   // TODO: Support cross-file checking when multiple files have edits.
   llvm::SmallVector<Check::Unit> units = {{{.consumer = &consumer,
                                             .value_stores = value_stores_.get(),
                                             .timings = nullptr,
-                                            .sem_ir = &sem_ir}}};
+                                            .sem_ir = &sem_ir,
+                                            .cpp_ast = &cpp_ast}}};
+
+  auto getter = [this]() -> const Parse::TreeAndSubtrees& {
+    return *tree_and_subtrees_;
+  };
   llvm::IntrusiveRefCntPtr<llvm::vfs::InMemoryFileSystem> fs =
       new llvm::vfs::InMemoryFileSystem;
   // TODO: Include the prelude.