The llvm_headers_do_not_build project needs to be excluded from the default build, otherwise it gets built (at least in Visual Studio 2013).

Thanks to chapuni200000 for help with this in IRC!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200321 91177308-0d34-0410-b5e6-96231b3b80d8