[Driver][Diagnostics] Make 'show option names' default for driver warnings

Currently, driver level warnings do not show option names (e.g. warning:
complain about foo [-Woption-name]) in a diagnostic unless
-fdiagnostics-show-option is explictly specified. OTOH, the driver by
default turn this option on for CC1. Change the logic to show option
names by default in the driver as well.

Differential Revision: https://reviews.llvm.org/D24516

rdar://problem/27300909

llvm-svn: 283827