msbuild integration: provide separate files for VS2010 and VS2012

The previous msbuild integration only worked if VS2010 was installed. This patch
renames the current integration to LLVM-vs2010 and adds LLVM-vs2012.

Differential Revision: http://llvm-reviews.chandlerc.com/D1614

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@190173 91177308-0d34-0410-b5e6-96231b3b80d8