Prior to commit 3c125c6d (VS: Support Visual Studio Clang Toolkit
identification, 2019-12-03, v3.17.0-rc1~341^2) using `-T ClangCL`
would work but `CMAKE_{C,CXX}_COMPILER` would be detected as `cl.exe`
even though `clang-cl.exe` is the actual compiler.  That commit
attempted to fix the detection by using `$(ClangClExecutable)`
as we do for LLVM-distributed toolsets, but that is not actually
defined.  Instead, look for `$(CLToolExe)` in the `PATH`.

Fixes: #20504