Build Qt WebEngine with Clang on Windows

Consider building Qt WebEngine with Clang on Windows. Upstream Chromium switched to clang as default toolchain on Windows a while ago:

https://groups.google.com/a/chromium.org/forum/#!topic/chromium-dev/Y3OEIKkdlu0

https://groups.google.com/a/chromium.org/d/msg/chromium-dev/01lcVxwEuLM/z7FybEQ_AgAJ

http://blog.llvm.org/2018/03/clang-is-now-used-to-build-chrome-for.html

(Upstream) Reasons given for the switch:

• clang works better with distributed build systems
• same toolchain for all platforms

Drawbacks:

• clang seems to be slower than MSVC
• clang binaries are reportedly bigger
• at time of writing, it still requires a full Visual Studio installation (uses VS headers + linker)

Things to find out:

• Which version of Clang is supported & where to get it?
• Are Clang and MSVC binary compatible enough that we can mix a VS build of Qt with a Clang build of Qt WebEngine?