Expose install_fatal_error_handler() through the C API.

I expose the API with some caveats:

- The C++ API involves a traditional void* opaque pointer for the fatal 
error callback.  The C API doesn’t do this.  I don’t think that the void* 
opaque pointer makes any sense since this is a global callback - there will 
only be one of them.  So if you need to pass some data to your callback, 
just put it in a global variable.

- The bindings will ignore the gen_crash_diag boolean.  I ignore it because 
(1) I don’t know what it does, (2) it’s not documented AFAIK, and (3) I 
couldn’t imagine any use for it.  I made the gut call that it probably 
wasn’t important enough to expose through the C API.

llvm-svn: 192864