Initialize output parameters

If the function returns true, it should
set all output paremeters, similar to Output::preflightElement, or we
have UB on code like:

```
void *SaveInfo;
if (io.preflightFlowElement(i, SaveInfo))
  io.postflightFlowElement(SaveInfo);
```

It's going to be detected by msan with:
-Xclang -enable-noundef-analysis -mllvm -msan-eager-checks=1

Differential Revision: https://reviews.llvm.org/D116826

llvm/lib/Support/YAMLTraits.cpp

@@ -629,7 +629,7 @@ void Output::endFlowSequence() {
   outputUpToEndOfLine(" ]");
 }
 
-bool Output::preflightFlowElement(unsigned, void *&) {
+bool Output::preflightFlowElement(unsigned, void *&SaveInfo) {
   if (NeedFlowSequenceComma)
     output(", ");
   if (WrapColumn && Column > WrapColumn) {
@@ -639,6 +639,7 @@ bool Output::preflightFlowElement(unsigned, void *&) {
     Column = ColumnAtFlowStart;
     output("  ");
   }
+  SaveInfo = nullptr;
   return true;
 }