[Refactoring] Consolidate handling of protection, storage_class, and userAttributes

They should be applied even for opaque declaration
[Refactorign] Consolidate `symtab`
[Refactoring] Fix ungagSpeculative() position
fix Issue 12555 - Incorrect error ungagging for speculatively instantiated class

`ungagSpeculative()` should do nothing for function local symbols.
Merge pull request #3452 from WalterBright/DIP29_6

implicit conversions for NewExp
Supply scope for addMember call
[Refactoring] AttribDeclaration::createNewScope
[Refactoring] AttribDeclaration::newScope
Generalize addMember and importAll for derived class of AttribDeclaration
Generalize semantic2 and semantic 3
Always delay static-if condition evaluation
fix Issue 12619 - Invalid warning for unused return value of debug memcpy
fix Issue 12649 - "discards return value" warning will cause ICE on function pointer call
More better fix for issue 5498

The root issue is in typeMerge function.
[Refactoring] Improve error propagation in arrayExpressionToCommonType
[Refactoring] Prevent storing ErrorExp in exps
fix Issue 8309 - ICE in typeMerge on 'void main(){auto x = [()=>1.0, ()=>1];}'
Merge pull request #3509 from AndrejMitrovic/Fix7747

Issue 7747 - Diagnostic should be informative for an inferred return type.
Merge pull request #3508 from AndrejMitrovic/Fix8101

Issue 8101 - Display candidate function overloads when function call fails.
Fix Issue 8101 - Fixup pull improving diagnostics when there are overloads.