[cfe-users] How to detect LLVM Visual Studio Toolset on Windows?

[Jeffrey Walton via cfe-users]

We took a bug report for LLVM Visual Studio Toolset on Windows. It
appears LLVM defines _MSC_VER but it cannot consume the same programs
that Microsoft's compilers can. We now have to figure out a way to
detect Clang in this configuration and work around its shortcomings.

The "Getting Started with the LLVM System using Microsoft Visual
Studio" points us to the FAQ at http://llvm.org/docs/FAQ.html. The FAQ
does not discuss how to detect the configuration or the preprocessor
macros that are in effect.

How do we detect LLVM Visual Studio Toolset on Windows?