[NFC] Add explicit initializer to PGOCtxProfReader's RootEntryCount

I found that on Clang versions before 11 we suppress the defaulted
constructor because RootEntryCount would not be initialized. This patch
adds an explicit initializer which should suppress this error.

Author: erichkeane

llvm/include/llvm/ProfileData/PGOCtxProfReader.h

@@ -92,7 +92,7 @@ class PGOCtxProfContext final : public internal::IndexNode {
 
   GlobalValue::GUID GUID = 0;
   SmallVector<uint64_t, 16> Counters;
-  const std::optional<uint64_t> RootEntryCount;
+  const std::optional<uint64_t> RootEntryCount{};
   CallsiteMapTy Callsites;
 
   PGOCtxProfContext(GlobalValue::GUID G, SmallVectorImpl<uint64_t> &&Counters,