vcruntime: Fix and enable RTTI

[thrimbor]

This fixes the lld: error: undefined symbol: const type_info::'vftable' linker error that would usually appear when enabling RTTI and linking a program that instantiates a class containing virtual methods, and enables RTTI. This is enough to give it a quick test:

class A {
  public:
  virtual ~A() {};
};

A a;

This is achieved by the following changes:

• Build the type_info class with -fno-rtti to prevent it from emitting a recursive reference to its vftable
• Implement the empty virtual destructor which acts as the key function
• Add a hidden constructor, because LLVM will optimize away the vftable if the class is not instantiable.

Removing const in __std_type_info_data is not necessary to get this working, but is required for when we eventually implement name demangling, which will require this structure to be modifiable at runtime. Some missing noexcept's were also fixed.

These changes are not enough to actually profit from having RTTI (such as using typeid and dynamic_cast), as these require functional SEH and C++ exception support to implement.

Fixes #237.

[JayFoxRox]

Please create an issue about this.

[thrimbor]

Done.

[JayFoxRox]

If we demangle at runtime, how will we know when to do it?
Wouldn't it have made more sense to keep it const and deal with it when implementing demangling?

[thrimbor]

If we demangle at runtime, how will we know when to do it?

MS demangles the name when type_info::name() is called and _UndecoratedName is NULL. It allocates a buffer for the string, demangles, and puts the address of the string in _UndecoratedName.

Wouldn't it have made more sense to keep it const and deal with it when implementing demangling?

I don't think there's a significant benefit in keeping or removing const at this moment - it just bothered me because it was incorrect.