Fix msbuild integration install script.

We previously failed to check whether the SUCCESS variable was set,
and would thus always exit with a failure if vs2012 didn't exist.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@192370 91177308-0d34-0410-b5e6-96231b3b80d8
Provide msbuild integration for vs2013.

Patch by Josh Samuel!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@192371 91177308-0d34-0410-b5e6-96231b3b80d8