Fix compatibility with clang-cl

[traversaro]

clang-cl is the Visual Studio-compatible version of clang (https://clang.llvm.org/docs/MSVCCompatibility.html).
This PR:

• Enables compilation of DART even for VS < 2019 if Clang is used (as clang-cl can be perfectly C++14 compatible even if the used MSVC version is not)
• Avoid to pass the /MP option to clang-cl that does not support it
• Adds a GitHub Actions job to compile DART with clang-cl (using in particular the ClangCl toolset shipped with MSVC)

[codecov]

Codecov Report

Merging #1509 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1509      +/-   ##
==========================================
- Coverage   58.18%   58.17%   -0.02%     
==========================================
  Files         414      414              
  Lines       30008    30008              
==========================================
- Hits        17459    17456       -3     
- Misses      12549    12552       +3     

Impacted Files	Coverage Δ
dart/dynamics/Skeleton.cpp	66.22% <0.00%> (-0.17%)	⬇️

[traversaro]

The macOS failure seem to be related to actions/runner-images#1811 .

[jslee02]

The macOS failure seem to be related to actions/virtual-environments#1811 .

It seems this build failure is unrelated to this PR. It'd make sense to fix in a separate CR.