devtoolset-9 has been out for a while so also look for it.
Differential Revision: https://reviews.llvm.org/D77420
// Non-Solaris is much simpler - most systems just go with "/usr".
if (SysRoot.empty() && TargetTriple.getOS() == llvm::Triple::Linux) {
// Yet, still look for RHEL devtoolsets.
+ Prefixes.push_back("/opt/rh/devtoolset-9/root/usr");
Prefixes.push_back("/opt/rh/devtoolset-8/root/usr");
Prefixes.push_back("/opt/rh/devtoolset-7/root/usr");
Prefixes.push_back("/opt/rh/devtoolset-6/root/usr");