Lazy deference underlying object for shared/weak/unique_ptr synthetic… (#67069)

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 #59: 0x00007ff4102e0660 liblldb.so.15`SymbolFileDWARF::CompleteType(this=<unavailable>, compiler_type=0x00007ffdd9829450) at SymbolFileDWARF.cpp:1567:25 [opt]
...
frame #67: 0x00007ff40fdf9bd4 liblldb.so.15`lldb_private::ValueObject::Dereference(this=0x0000022bb5dfe980, error=0x00007ffdd9829970) at ValueObject.cpp:2672:41 [opt]
frame #68: 0x00007ff41011bb0a liblldb.so.15`(anonymous namespace)::LibStdcppSharedPtrSyntheticFrontEnd::Update(this=0x000002298fb943...