[memprof] Make HeapProfileRecords optional

memprof::AllMemProfData has become home to multiple types of memprof
data -- heap profile and static data profile.  When we write test
cases for static data profile in YAML, we do not want to include empty
heap profile.  That would just add visual clutter.

This patch makes HeapProfileRecords optional.

Author: kazutakahirata

llvm/include/llvm/ProfileData/MemProfYAML.h

@@ -263,7 +263,8 @@ template <> struct MappingTraits<memprof::YamlDataAccessProfData> {
 
 template <> struct MappingTraits<memprof::AllMemProfData> {
   static void mapping(IO &Io, memprof::AllMemProfData &Data) {
-    Io.mapRequired("HeapProfileRecords", Data.HeapProfileRecords);
+    if (!Io.outputting() || !Data.HeapProfileRecords.empty())
+      Io.mapOptional("HeapProfileRecords", Data.HeapProfileRecords);
     // Map data access profiles if reading input, or if writing output &&
     // the struct is populated.
     if (!Io.outputting() || !Data.YamlifiedDataAccessProfiles.isEmpty())