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.
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.