title: VSCode C/C++ Intellisense not working with CMake project
body: <p>I'm trying to write a tool using libTooling. I got it set up so that it compiles with the example from the LLVM Documentation. However C/C++ Intellisense seems to not work with CMake projects.</p>
  
  <p>My tool is located in:</p>
  
  <p><code>&lt;project&gt;/clang-tools-extra/mytool</code></p>
  
  <p>Now the C/C++ extension tries to read the compile_config.json and tells me <code>&lt;project&gt;/build/compile_config.json</code> could not be found, using <code>includePath</code> from <code>c_cpp_properties.json</code> instead.</p>
  
  <p>I tried adding the include paths manually in my workspace settings:</p>
  
  <pre><code>{
      "C_Cpp.default.includePath": [
          "../../",
          "../../clang/Frontend/",
          "../../clang/Tooling/",
          "../../llvm/Support/"
      ],
      "C_Cpp.default.browse.path": [
          "../.."
      ]
  }
  </code></pre>
  
  <p>Or in a file <code>c_cpp_properties.json</code>. But it still searches for the includes in the wrong place. E.g. the include:</p>
  
  <pre class="lang-c prettyprint-override"><code>#include "llvm/Support/CommandLine.h"
  </code></pre>
  
  <p>It tries to find in <code>&lt;project&gt;/llvm/include/llvm/Support/CommandLine.h</code>. So apparently it reads something from command_config.json even though it says it can't find it (while it is there), but the wrong thing. It shouldn't add <code>llvm/include</code> at all.</p>