[feature] Detect if running clang compiler on Windows

[uilianries]

Changelog: Feature: conan.tools.microsoft.is_clang_cl detects if running Clang on Windows
Docs: conan-io/docs#2774

close #12175

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[memsharded]

I am afraid this is no longer valid. The clangCL is nothing specific anymore, and is not part of the new msvc compiler either, the right compiler is clang, with generator Visual Studio.

Check: #11492

[uilianries]

Closing this one. With #11492 is clear that a generic check for CLang on Windows will not work for many cases. So far, 3% of CCI recipes need to validate clang on Windows, but we don't have a clear situation about which clang and its version. It will be better analyzing each case there and consider the new proposal from #11492.

[SpaceIm]

Is it more clear now that #11492 has been merged? 3% of CCI recipes try to manage clang CL or a mingw clang flavor, but many more are broken and would need better detection of clang CL.

It would also be nice to have a is_mingw() (because it can be gcc targeting Windows, or a clang flavor targeting windows).