[libcxx] r196411 - Give all members of exception types default visibility. Lack of this is causing some illegal code relocations rare and hard to reproduce cases.

[Rafael Espíndola via cfe-commits]

> I'm not sure about GCC.  Note that there is no linkage for -frtti,
> since the type info is deferred to the TU with the vtable.

Yes, that is what I mean. It is odd that -frtti changes us from "this
is not available anywhere, use linkonce_odr" to "it is available
elsewhere, use an external declaration".

Cheers,
Rafael