[Driver] fix broken test

Summary:
Fixes test from r344941 which was broken on Windows. We want to check
the selected toolchain rather than the found toolchain anyways.

Reviewers: srhines, danalbert

Reviewed By: srhines

Subscribers: cfe-commits, bogner, pirama

Differential Revision: https://reviews.llvm.org/D53529

llvm-svn: 344958