Driver: Add gcc search path for RHEL devtoolset-9

[merge_guards_bot]

Unit tests: pass. 62282 tests passed, 0 failed and 827 were skipped.

clang-tidy: pass.

clang-format: pass.

Build artifacts: diff.json, clang-tidy.txt, clang-format.patch, CMakeCache.txt, console-log.txt, test-results.xml

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clang/lib/Driver/ToolChains/Gnu.cpp

for (llvm::vfs::directory_iterator LI = D.getVFS().dir_begin(PrefixDir, EC),
       Prefixes.push_back(CandidatePrefix);
     }
     return;
   }
 
   // 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");
     Prefixes.push_back("/opt/rh/devtoolset-4/root/usr");
     Prefixes.push_back("/opt/rh/devtoolset-3/root/usr");
     Prefixes.push_back("/opt/rh/devtoolset-2/root/usr");
   }
   Prefixes.push_back(SysRoot.str() + "/usr");