Make LLVM_MSC_PREREQ() compatible with all MSVC versions

The defined() preprocessor expansion wasn't working out on the lld builder.

Also update the documentation to cover another Visual Studio release versioning
convention.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@198158 91177308-0d34-0410-b5e6-96231b3b80d8