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
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