Add LLVM -clang to projucer, as a toolset for Windows

[DaveH]

I just discovered I can add LLVM to projucer just by simply adding the toolset to exporter properties function. You may have to be careful with the upper case characters, but this seems to work.
btw there’s a function for each VS version…

void createExporterProperties (PropertyListBuilder& props) override
    {
        static const char* toolsetNames[] = { "v140", "v140_xp", "v141", "v141_xp", "v142", "v143", "LLVM (clang-cl)"};
        const var toolsets[]              = { "v140", "v140_xp", "v141", "v141_xp", "v142", "v143", "ClangCL" };
        addToolsetProperty (props, toolsetNames, toolsets, numElementsInArray (toolsets));

        MSVCProjectExporterBase::createExporterProperties (props);
    }