[LLDB] Explicitly declare constructor in `PersistentExpressionState`

[laszlokindrat]

Summary

It seems that when trying to link the lldb library explicitly, the inlined default constructor cannot find the vtable for the class. This patch fixes this by explicitly declaring a default constructor in PersistentExpressionState, and providing the definition in the source file.

[wallace]

thanks!

lldb/include/lldb/Expression/ExpressionVariable.h

 
 class PersistentExpressionState
     : public ExpressionVariableList,
       public llvm::RTTIExtends<PersistentExpressionState, llvm::RTTIRoot> {
 public:
   /// LLVM RTTI support
   static char ID;
 
+  PersistentExpressionState();
+
   virtual ~PersistentExpressionState();
 
   virtual lldb::ExpressionVariableSP
   CreatePersistentVariable(const lldb::ValueObjectSP &valobj_sp) = 0;
 
   virtual lldb::ExpressionVariableSP
   CreatePersistentVariable(ExecutionContextScope *exe_scope,
                            ConstString name, const CompilerType &type,

lldb/source/Expression/ExpressionVariable.cpp

if (byte_size && *byte_size) {
     return const_cast<uint8_t *>(
         m_frozen_sp->GetDataExtractor().GetDataStart());
   }
   return nullptr;
 }
 
 char PersistentExpressionState::ID;
 
+PersistentExpressionState::PersistentExpressionState() = default;
+
 PersistentExpressionState::~PersistentExpressionState() = default;
 
 lldb::addr_t PersistentExpressionState::LookupSymbol(ConstString name) {
   SymbolMap::iterator si = m_symbol_map.find(name.GetCString());
 
   if (si != m_symbol_map.end())
     return si->second;
   else