[memprof] Use ListSeparator (NFC)

ListSeparator from StringExtras.h is essentially the same as
FieldSeparator being removed in this patch.  ListSeparator returns the
empty string on the first use via "operator StringRef()".  It returns
", " on subsequent uses.

Author: kazutakahirata

llvm/lib/Transforms/IPO/MemProfContextDisambiguation.cpp

@@ -896,21 +896,6 @@ struct DenseMapInfo<IndexCall>
 
 namespace {
 
-struct FieldSeparator {
-  bool Skip = true;
-  const char *Sep;
-
-  FieldSeparator(const char *Sep = ", ") : Sep(Sep) {}
-};
-
-raw_ostream &operator<<(raw_ostream &OS, FieldSeparator &FS) {
-  if (FS.Skip) {
-    FS.Skip = false;
-    return OS;
-  }
-  return OS << FS.Sep;
-}
-
 // Map the uint8_t alloc types (which may contain NotCold|Cold) to the alloc
 // type we should actually use on the corresponding allocation.
 // If we can't clone a node that has NotCold+Cold alloc type, we will fall
@@ -2784,9 +2769,9 @@ void CallsiteContextGraph<DerivedCCG, FuncTy, CallTy>::ContextNode::print(
     OS << "\t\t" << *Edge << "\n";
   if (!Clones.empty()) {
     OS << "\tClones: ";
-    FieldSeparator FS;
+    ListSeparator LS;
     for (auto *Clone : Clones)
-      OS << FS << Clone;
+      OS << LS << Clone;
     OS << "\n";
   } else if (CloneOf) {
     OS << "\tClone of " << CloneOf << "\n";