[clang] Update lit config in utils/perf-training

This diff replaces --driver-mode=cpp in
utils/perf-training/order-files.lit.cfg and
utils/perf-training/lit.cfg with --driver-mode=g++.
clang --driver-mode=cpp will call the preprocessor and will not
trigger compilation.

Differential revision: https://reviews.llvm.org/D28269

llvm-svn: 290936