Add support for detection of devtoolset-8

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 e5ac385f)
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

Patch By: Radu-Adrian Popescu

Reviewers: tstellar, fedor.sergeev

Reviewed By: tstellar

Subscribers: jdoerfert, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D59987

llvm-svn: 358002