title: How to choose the LLVM version clang is using?
body: <p>Maybe it's too obvious, but I don't see the answer yet:</p>
  
  <p>I try to compile my project under Ubuntu with clang, but unfortunately llvm is crashing with a stack dump.</p>
  
  <pre><code>$ clang --version
  Ubuntu clang version 3.0-6ubuntu3 (tags/RELEASE_30/final) (based on LLVM 3.0)
  Target: i386-pc-linux-gnu
  Thread model: posix
  </code></pre>
  
  <p>I would like to give LLVM 2.9 a try, installed it, but how can I choose it? E.g.:</p>
  
  <pre><code>$ ll /usr/bin/llvm-ar*
  lrwxrwxrwx 1 root root 27 Sep 12 16:43 /usr/bin/llvm-ar -&gt; ../lib/llvm-3.1/bin/llvm-ar*
  lrwxrwxrwx 1 root root 27 May 21  2012 /usr/bin/llvm-ar-2.9 -&gt; ../lib/llvm-2.9/bin/llvm-ar*
  lrwxrwxrwx 1 root root 27 Aug 27 20:31 /usr/bin/llvm-ar-3.0 -&gt; ../lib/llvm-3.0/bin/llvm-ar*
  lrwxrwxrwx 1 root root 27 Aug 28 17:49 /usr/bin/llvm-ar-3.1 -&gt; ../lib/llvm-3.1/bin/llvm-ar*
  $ ll /usr/bin/clang*
  -rwxr-xr-x 1 root root 12686228 Apr 25  2012 /usr/bin/clang*
  lrwxrwxrwx 1 root root        5 Apr 25  2012 /usr/bin/clang++ -&gt; clang*
  </code></pre>
  
  <p>Clang as a command exists only once, and I didn't see a command line option of it to choose the llvm backend. Am I missing something?</p>