Dsymbol's affected by this AttribDeclaration
Create a new scope if one or more given attributes are different from the sc's. If the returned scope != sc, the caller should pop the scope after it used.
A hook point to supply scope for members. addMember, setScope, importAll, semantic, semantic2 and semantic3 will use this.
Attribute declaring whether an external aggregate should be mangled as a struct or class in C++, e.g. extern(C++, struct) class C { ... }. This is required for correct name mangling on MSVC targets, see cppmanglewin.d for details.
extern(C++, <cppmangle>) <decl...>