Unify symbol reference handling

[xlauko]

Prerequisites

• I have read the documentation and the proposed feature is not implemented.

Description

• remove SSA value from DeclRefOp and make it take FlatSymbolRefAttr instead
• remove obsolete graph regions
• unify GlobalRef and DeclRef to single operation as they will behave the same now
• rename RecordMemberOp to MemberRefOp
• let RecordType use SymbolRefParameter instead of a pure StringRefParameter
• let EnumType use SymbolRefParameter instead of a pure StringRefParameter
• let TypedefType use SymbolRefParameter instead of a pure StringRefParameter
• resurrect vast-query to test symbol API
• remove util/Symbols.hpp in favor of generic SymbolTable lookup
• fix VarDeclOp::getDeclContextKind to use vast symbol tables (use get_effective_symbol_table_for)
• replace SymbolTable::getSymbolAttrName() with symbol_attr_name
• create decl ref interface to unify all symbol referencing operations