Define LLVM_MSC_PREREQ() macro to simplify _MSC_VER checks

Includes documentation mapping MSC version numbers to the more familiar Visual
Studio releases.

Cleanup only to simplify upcoming C++11 / MSVC 2013 changes.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@198141 91177308-0d34-0410-b5e6-96231b3b80d8
Enable deleted functions and explicit conversions in MSVC 2013

Also prospectively enable static_assert as the documentation suggests it's been
available since MSVC 2010. Let's see if the build servers agree.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@198142 91177308-0d34-0410-b5e6-96231b3b80d8