Add support for VS2015 Clang/C2 projects

Please add support for generating VS2015 projects that use "Clang with Microsoft CodeGen" (aka Clang/C2) out-of-the-box. (here and here for more info).

Right now, setting the "platform toolset" requires to either open each project individually and set "Configuration Properties" -> "General" -> "Platform toolset" to "Clang with Microsoft CodeGen" (eg. configuration window) or to patch the generated .vcxproj files to set <PlatformToolset>v140_clang_3_7</PlatformToolset>.

Using Clang/C2 also requires:

• changing all compiler options ("Additional Options") to clang's "-option" style (as opposed to CL's "/option" style).
• changing the "Debug Information Format" to DWARF2 (-g2 -gdwarf-2)
• changing the "Warning Level" to either -W0 or -Wall.

Regards.