[docs] Add a note that the Visual Studio C++ tools are required
[docs] Add a note that the Visual Studio C++ tools are required

Watching new contributors trying to build LLVM on Windows, one of the
very common failure modes was getting a version of Visual Studio
that did not have a C++ compiler for CMake to put up. Trying to create
a C++ project in Visual Studio will cause Visual Studio to go and
download the C++ tools.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@260049 91177308-0d34-0410-b5e6-96231b3b80d8

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