[Perf-training] Adding support for tests to skip the clang driver

This patch adds a new set of substitutions to the lit run lines for order files and PGO generation which run the clang driver to get the cc1 command, then execute the cc1 command directly. This allows the scripts to bypass profiling the clang driver over and over again.

The approach in this patch was discussed via IRC with Sean Silvas.

Special thanks to Daniel Dunbar whose out-of-tree code I liberally plagiarized.

llvm-svn: 263997