Add AtosSymbolizer and DlAddrSymbolizer as fallbacks for OS X

This patch changes the symbolizer chain on OS X (which currently only uses 1
symbolizer at most) to use this behavior:

* By default, use LLVMSymbolizer -> DlAddrSymbolizer.
* If the llvm-symbolizer binary is not found, use AtosSymbolizer 
    -> DlAddrSymbolizer.
* If the user specifies ASAN_SYMBOLIZER_PATH=.../atos, then use AtosSymbolizer
    -> DlAddrSymbolizer.
* If neither llvm-symbolizer or atos is found, or external symbolication is
    disabled with ASAN_SYMBOLIZER_PATH="", use DlAddrSymbolizer.

Reviewed at http://reviews.llvm.org/D8285

llvm-svn: 232908