VS: Fix detection of clang-cl with -T llvm
When using a VS generator with -T llvm, MSBuild relies on the "LLVM
Compiler Toolchain" VS Extension. This does not put clang-cl in the
PATH inside the build, and LLVM no longer provides a cl replacement
either. Therefore we need another way to extract the path to the
CMAKE_{C,CXX}_COMPILER. Fortunately the LLVM VS integration provides
a $(ClangClExecutable) macro we can reference to get the path.
Fixes: #18983 (closed)