This doesn't enforce anything.  But we can actually test this one!

`extern(C++) void verror(Loc loc, const(char)* format, void* ap, const(char)* p1 = null, const(char)* p2 = null, const(char)* header = "Error: ")`
has mangling `"?verror@@YAXULoc@@PBDPAX111@Z"`, but
`extern(C++) void verror(Loc loc, const(char)* format, char* ap, const(char)* p1 = null, const(char)* p2 = null, const(char)* header = "Error: ")`
has the desired mangling of `"?verror@@YAXULoc@@PBDPAD111@Z"`

If they are both renamed to the same mangling, and one has a body while one is extern, we should get a link error if the renamed one is incorrect.

eg
```
extern(C++) extern void a(char *);
pragma(mangle, whatever_a_mangles_to)
extern(C++) void b(void *) {}
void main() { a(null); }
```