We recently added support for -mtune. This patch adds /tune: so we can specify the tune CPU from clang-cl. MSVC doesn't support this but icc does.
I didn't know you could spell it as /clang:-mtune. ICC has /tune: according to this documentation https://software.intel.com/content/www/us/en/develop/documentation/cpp-compiler-developer-guide-and-reference/top/compiler-reference/compiler-options/compiler-option-details/code-generation-options/mtune-tune.html