Test agains Clang for ClangCL toolset

[Psy-Kai]

Changelog: Fix: Let CMake generator generate code for checking against "ClangCL" msvc toolset.
Docs: Omit

Without that fix the generated conanbuildinfo.cmake would check for compiler and just fail because cmake detected clang but the code would compare with "MSVC".

Fixes #5896
May also fix the #1839 for Visual Studio 2019

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[memsharded]

Approved, but please take into account these hints for Conan direction:

• The cmake generator will be replaced by the CMakeToolchain and CMakeDeps (forked from cmake_find_package_multi generator), that are still experimental, but planned to replace it in Conan 2.0
• There is a mechanism to skip the compiler check, just as a tip to unblock: define the CMake variable CONAN_DISABLE_CHECK_COMPILER