We added basic support for clang-cl recently, but we don't handle-Xclang, which can be used to pass options down to the clang frontend (vs. the clang-cl frontend which parses MSVC options). We use this in Firefox builds to pass down GCC-style dependency options, like:
-Xclang -MP -Xclang -dependency-file -Xclang .deps/lz4.obj.pp -Xclang -MT -Xclang lz4.obj

Currently sccache refuses to cache these compiles because it thinks the arguments to -Xclang that do not start with a dash are input source files, and it will not cache a compile with multiple source files. At the very least we need it to understand that -Xclang takes an argument so that it parses the commandline correctly.

A more thorough followup would be to correctly detect clang-cl as clang-cl, and parse its commandlines with MSVC arguments but also accumulate -Xclang arguments into a separate list and then parse them with the clang argument parser, which might be a bit hairy. Otherwise we won't properly cache compiles like the example above, where we're instructing clang-cl to output a dependency file.