Driver: Don't mix system tools with devtoolset tools on RHEL

Summary:
On RHEL, devtoolset provides a more up-to-date toolchain than the base
install, and we want to make sure all the tools use are from the same
toolchain.

Reviewers: rsmith, bruno

Reviewed By: bruno

Subscribers: bruno, cfe-commits

Differential Revision: https://reviews.llvm.org/D34848

llvm-svn: 336037