Lazy deference underlying object for shared/weak/unique_ptr synthetic provider

We noticed some performance issue while in lldb-vscode for grabing the name of the SBValue.
Profiling shows SBValue::GetName() can cause synthetic children provider of shared/unique_ptr
to deference underlying object and complete it type.

This patch lazily moves the dereference from synthetic child provider's Update() method to
GetChildAtIndex() so that SBValue::GetName() won't trigger the slow code path.

Here is the culprit slow code path:
```
...
frame llvm#59: 0x00007ff4102e0660 liblldb.so.15`SymbolFileDWARF::CompleteType(this=<unavailable>, compiler_type=0x00007ffdd9829450) at SymbolFileDWARF.cpp:1567:25 [opt]
...
frame llvm#67: 0x00007ff40fdf9bd4 liblldb.so.15`lldb_private::ValueObject::Dereference(this=0x0000022bb5dfe980, error=0x00007ffdd9829970) at ValueObject.cpp:2672:41 [opt]
frame llvm#68: 0x00007ff41011bb0a liblldb.so.15`(anonymous namespace)::LibStdcppSharedPtrSyntheticFrontEnd::Update(this=0x000002298fb94380) at LibStdcpp.cpp:403:40 [opt]
frame llvm#69: 0x00007ff41011af9a liblldb.so.15`lldb_private::formatters::LibStdcppSharedPtrSyntheticFrontEndCreator(lldb_private::CXXSyntheticChildren*, std::shared_ptr<lldb_private::ValueObject>) [inlined] (anonymous namespace)::LibStdcppSharedPtrSyntheticFrontEnd::LibStdcppSharedPtrSyntheticFrontEnd(this=0x000002298fb94380, valobj_sp=<unavailable>) at LibStdcpp.cpp:371:5 [opt]
...
frame llvm#78: 0x00007ff40fdf6e42 liblldb.so.15`lldb_private::ValueObject::CalculateSyntheticValue(this=0x000002296c66a500) at ValueObject.cpp:1836:27 [opt]
frame llvm#79: 0x00007ff40fdf1939 liblldb.so.15`lldb_private::ValueObject::GetSyntheticValue(this=<unavailable>) at ValueObject.cpp:1867:3 [opt]
frame llvm#80: 0x00007ff40fc89008 liblldb.so.15`ValueImpl::GetSP(this=0x0000022c71b90de0, stop_locker=0x00007ffdd9829d00, lock=0x00007ffdd9829d08, error=0x00007ffdd9829d18) at SBValue.cpp:141:46 [opt]
frame llvm#81: 0x00007ff40fc7d82a liblldb.so.15`lldb::SBValue::GetSP(ValueLocker&) const [inlined] ValueLocker::GetLockedSP(this=0x00007ffdd9829d00, in_value=<unavailable>) at SBValue.cpp:208:21 [opt]
frame llvm#82: 0x00007ff40fc7d817 liblldb.so.15`lldb::SBValue::GetSP(this=0x00007ffdd9829d90, locker=0x00007ffdd9829d00) const at SBValue.cpp:1047:17 [opt]
frame llvm#83: 0x00007ff40fc7da6f liblldb.so.15`lldb::SBValue::GetName(this=0x00007ffdd9829d90) at SBValue.cpp:294:32 [opt]
...
```

Differential Revision: https://reviews.llvm.org/D159542

Author: Jeffrey Tan

lldb/source/Plugins/Language/CPlusPlus/LibStdcpp.cpp

@@ -377,9 +377,16 @@ lldb::ValueObjectSP
 LibStdcppSharedPtrSyntheticFrontEnd::GetChildAtIndex(size_t idx) {
   if (idx == 0)
     return m_ptr_obj->GetSP();
-  if (idx == 1)
-    return m_obj_obj->GetSP();
-
+  if (idx == 1) {
+    if (m_ptr_obj && !m_obj_obj) {
+      Status error;
+      ValueObjectSP obj_obj = m_ptr_obj->Dereference(error);
+      if (error.Success())
+        m_obj_obj = obj_obj->Clone(ConstString("object")).get();
+    }
+    if (m_obj_obj)
+      return m_obj_obj->GetSP();
+  }
   return lldb::ValueObjectSP();
 }
 
@@ -397,14 +404,7 @@ bool LibStdcppSharedPtrSyntheticFrontEnd::Update() {
     return false;
 
   m_ptr_obj = ptr_obj_sp->Clone(ConstString("pointer")).get();
-
-  if (m_ptr_obj) {
-    Status error;
-    ValueObjectSP obj_obj = m_ptr_obj->Dereference(error);
-    if (error.Success()) {
-      m_obj_obj = obj_obj->Clone(ConstString("object")).get();
-    }
-  }
+  m_obj_obj = nullptr;
 
   return false;
 }

lldb/source/Plugins/Language/CPlusPlus/LibStdcppUniquePointer.cpp

@@ -108,14 +108,7 @@ bool LibStdcppUniquePtrSyntheticFrontEnd::Update() {
     if (del_obj)
       m_del_obj = del_obj->Clone(ConstString("deleter")).get();
   }
-
-  if (m_ptr_obj) {
-    Status error;
-    ValueObjectSP obj_obj = m_ptr_obj->Dereference(error);
-    if (error.Success()) {
-      m_obj_obj = obj_obj->Clone(ConstString("object")).get();
-    }
-  }
+  m_obj_obj = nullptr;
 
   return false;
 }
@@ -128,8 +121,17 @@ LibStdcppUniquePtrSyntheticFrontEnd::GetChildAtIndex(size_t idx) {
     return m_ptr_obj->GetSP();
   if (idx == 1 && m_del_obj)
     return m_del_obj->GetSP();
-  if (idx == 2 && m_obj_obj)
-    return m_obj_obj->GetSP();
+  if (idx == 2) {
+    if (m_ptr_obj && !m_obj_obj) {
+      Status error;
+      ValueObjectSP obj_obj = m_ptr_obj->Dereference(error);
+      if (error.Success()) {
+        m_obj_obj = obj_obj->Clone(ConstString("object")).get();
+      }
+    }
+    if (m_obj_obj)
+      return m_obj_obj->GetSP();
+  }
   return lldb::ValueObjectSP();
 }