Add support for detection of devtoolset-8

[rpopescu]

Summary

The current llvm/clang et al. project can be built with the latest developer toolset (devtoolset-8) on RHEL, which provides GCC 8.2.1.
However, the result compiler will not identify this toolset itself when compiling programs, which is of course not desirable.

After the patch - which simply adds the name of the developer toolset to the existing list - it gets identified and selected, as shown below:

[bamboo@bamboo llvm-project]$ clang -v
clang version 9.0.0 (https://github.com/llvm/llvm-project.git e5ac385fb1ffa4bd3875ea6a4d24efdbd7814572)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /home/bamboo/llvm/bin
Found candidate GCC installation: /opt/rh/devtoolset-4/root/usr/lib/gcc/x86_64-redhat-linux/5.2.1
Found candidate GCC installation: /opt/rh/devtoolset-7/root/usr/lib/gcc/x86_64-redhat-linux/7
Found candidate GCC installation: /opt/rh/devtoolset-8/root/usr/lib/gcc/x86_64-redhat-linux/8
Found candidate GCC installation: /usr/lib/gcc/x86_64-redhat-linux/4.8.2
Found candidate GCC installation: /usr/lib/gcc/x86_64-redhat-linux/4.8.5
Selected GCC installation: /opt/rh/devtoolset-8/root/usr/lib/gcc/x86_64-redhat-linux/8
Candidate multilib: .;@m64
Candidate multilib: 32;@m32
Selected multilib: .;@m64

[tstellar]

LGTM.

[tstellar]

Do you have commit access?

[rpopescu]

In D59987#1454422, @tstellar wrote:

Do you have commit access?

Hi Tom, I don't think that I do. I have created the account just before submitting the patch. Should I request it, or?

[tstellar]

In D59987#1454543, @rpopescu wrote:

In D59987#1454422, @tstellar wrote:

Do you have commit access?

Hi Tom, I don't think that I do. I have created the account just before submitting the patch. Should I request it, or?

I'll commit it for you.

cfe/trunk/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-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");
 }