fix exception_tracer build on macOS

Summary:
exception_tracer is currently gated on FOLLY_USE_SYMBOLIZER, and when
I enabled it, I didn't notice the CMake build of Folly on macOS
stopped working.

Fix it by adding correct conditionals, and removing the removed
std::unexpected_handler.

Reviewed By: yfeldblum, lnicco

Differential Revision: D23690777

fbshipit-source-id: 76ada7911ac33de3b1e8b0a73cebf3c2d04a58ef

folly/experimental/exception_tracer/ExceptionAbi.h

@@ -30,7 +30,8 @@ namespace __cxxabiv1 {
 struct __cxa_exception {
   std::type_info* exceptionType;
   void (*exceptionDestructor)(void*);
-  std::unexpected_handler unexpectedHandler;
+  void (*unexpectedHandler)(); // std::unexpected_handler has been removed from
+                               // C++17.
   std::terminate_handler terminateHandler;
   __cxa_exception* nextException;
 

folly/experimental/exception_tracer/ExceptionTracer.cpp

@@ -66,6 +66,7 @@ void printExceptionInfo(
   }
   out << " (" << info.frames.size()
       << (info.frames.size() == 1 ? " frame" : " frames") << ")\n";
+#if FOLLY_HAVE_ELF && FOLLY_HAVE_DWARF
   try {
     size_t frameCount = info.frames.size();
 
@@ -92,6 +93,7 @@ void printExceptionInfo(
   } catch (...) {
     out << "\n !!! caught unexpected exception\n";
   }
+#endif
 }
 
 namespace {