Remove the SharedVariant pointer from StringData.

We only use it to lazily grab the hashcode.  So just grab the hashcode
eagerly when we create the string.

hphp/runtime/base/string_data.cpp

@@ -300,10 +300,9 @@ HOT_FUNC
 StringData::StringData(SharedVariant *shared)
   : _count(0) {
   assert(shared && size_t(shared->stringLength()) <= size_t(MaxSize));
-  m_hash = 0;
+  m_hash = shared->stringHash();
   m_len = shared->stringLength();
   m_cdata = shared->stringData();
-  m_big.shared = shared;
   m_big.cap = m_len | IsShared;
 }
 
@@ -914,8 +913,8 @@ int StringData::compare(const StringData *v2) const {
 
 HOT_FUNC
 strhash_t StringData::hashHelper() const {
-  strhash_t h = isShared() ? m_big.shared->stringHash() :
-                             hash_string_inline(m_data, m_len);
+  assert(!isShared());
+  strhash_t h = hash_string_inline(m_data, m_len);
   assert(h >= 0);
   m_hash |= h;
   return h;