[Refactoring] Clean up syntaxCopy functions

Dsymbol, Statement, Expression, Type, and Initializer.
Merge pull request #3886 from 9rnsr/refactor_syntaxCopy

[Refactoring] Clean up syntaxCopy functions
[Refactoring] Reduce unnecessary cast from AggregateDeclaration to StructDeclaration
fix Issue 13321 - Wrong goto skips declaration error
Add VectorExp handling in CTFE interpreter to fix compilable/test8543.d