fall back to .debug_info scan in fatal signal handler

fall back to .debug_info scan in fatal signal handler

Summary:
We've found clang might be generating incomplete `.debug_aranges`,
while falling back to linear `.debug_info` scan is too expensive and shouldn't
be used by default, we can afford doing that in fatal signal handler.

Also optimize `exception_tracer::printExceptionInfo()` to avoid `LocationInfo`
resolution if `NO_FILE_AND_LINE` is used.

Reviewed By: luciang, ot

Differential Revision: D4020989

fbshipit-source-id: 84172208736b224c19206da48bcb3b5c0b2c67d0