[UWP] Switch the compiler to Clang for better performance

[mattleibow]

Description

Google has indicated that they only optimize for the clang compiler:

• Windows: https://groups.google.com/forum/#!topic/skia-discuss/ZZ7Ol3alB-I
• Linux: https://groups.google.com/forum/#!topic/skia-discuss/P4tg5eBVIiU

VS bug #730674

[dtoppani-twist]

I see work to use clang on linux to get more performance - is it possible to do so on Windows or there are specific blockers?