ci: Update LLVM/Clang to 16.0 in nightly CI jobs on Windows
Also enable C++ modules tests with LLVM/Clang 16.0 on Windows. This works only with the GNU-like front-end and the Ninja generator.
System updates will be applied on Apr 24th between 7am and 9am, EST (UTC-05:00). This site will have intermittent downtime during that time.
Also enable C++ modules tests with LLVM/Clang 16.0 on Windows. This works only with the GNU-like front-end and the Ninja generator.