Fix some more misuses of LLVM's YAML parser. (#15119)

https://reviews.llvm.org/D44317 should prevent this from happening
again.

Author: jrose-apple

lib/IDE/APIDigesterData.cpp

@@ -309,7 +309,7 @@ serializeDiffItem(llvm::BumpPtrAllocator &Alloc,
 #define DIFF_ITEM_KEY_KIND_STRING(NAME) StringRef NAME;
 #define DIFF_ITEM_KEY_KIND_INT(NAME) Optional<int> NAME;
 #include "swift/IDE/DigesterEnums.def"
-  for (auto Pair : *Node) {
+  for (auto &Pair : *Node) {
     switch(parseKeyKind(getScalarString(Pair.getKey()))) {
 #define DIFF_ITEM_KEY_KIND_STRING(NAME)                                       \
     case DiffItemKeyKind::KK_##NAME:                                          \

lib/Serialization/Serialization.cpp

@@ -4347,7 +4347,7 @@ class YamlGroupInputParser {
     if (!MapNode) {
       return true;
     }
-    for (auto Pair : *MapNode) {
+    for (auto &Pair : *MapNode) {
       auto *Key = dyn_cast_or_null<llvm::yaml::ScalarNode>(Pair.getKey());
       auto *Value = dyn_cast_or_null<llvm::yaml::SequenceNode>(Pair.getValue());
 

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

@@ -939,7 +939,7 @@ SDKNode* SDKNode::constructSDKNode(SDKContext &Ctx,
   SDKNodeInitInfo Info(Ctx);
   NodeVector Children;
 
-  for (auto Pair : *Node) {
+  for (auto &Pair : *Node) {
     switch(parseKeyKind(GetScalarString(Pair.getKey()))) {
     case KeyKind::KK_kind:
       Kind = parseSDKNodeKind(GetScalarString(Pair.getValue()));