swift-api-digester: initialize vector with a reasonable capacity.

In Jenkins bot, we've seen "exception std::length_error: vector" for
api-digester tests. Tentatively, this patch initializes a major vector with a
reasonably large size, which may avoid its wild capacity growth.

tools/swift-api-digester/swift-api-digester.cpp

@@ -166,10 +166,12 @@ typedef std::map<NodePtr, NodePtr> ParentMap;
 typedef std::vector<NodePtr> NodeVector;
 
 class SDKContext {
+  unsigned SIZE = 1024 * 1024;
   llvm::StringSet<> TextData;
   std::vector<std::unique_ptr<SDKNode>> OwnedNodes;
 
 public:
+  SDKContext() : OwnedNodes(SIZE) {}
   SDKNode* own(SDKNode *Node) {
     assert(Node);
     OwnedNodes.emplace_back(Node);