Re-land r219354: Use llvm-symbolizer to symbolize LLVM/Clang crash dumps.

In fact, symbolization is now expected to work only on Linux and
FreeBSD/NetBSD, where we have dl_iterate_phdr and can learn the
main executable name without argv0 (it will be possible on BSD systems
after http://reviews.llvm.org/D5693 lands). #ifdef-out the code for
all the rest Unix systems.

Reviewed in http://reviews.llvm.org/D5610