I've added Apple-only behavior that looks for libstdc++ in the same process...

I've added Apple-only behavior that looks for libstdc++ in the same process and if found, checks the string stored in <stdexcept> exception objects to see if it is the gcc empty string singleton before manipulating the reference count.  This is done so that if such an exception is created with a zero-length string in libstdc++, libc++abi won't try to delete the memory.  This is part of a ongoing process to make libc++ exceptions ABI-compatible with libstdc++-4.2 exceptions, to the point that each library can catch exceptions thrown by the other.  If other parties would also like this behavior, the #if __APPLE__ can be broadened.

llvm-svn: 164170