LLVM+Clang: upgrade to version 16.0.6

[agarny]

While working on issue #368, we went back to version 14.0.3 of LLVM+Clang which was the last version of LLVM+Clang that we were able to build on Windows/Linux ARM. However, version 14.0.6 got released since we originally used version 14.0.3, so we "upgraded" to it.

In fact, versions 15.0.7, 16.0.6, 17.0.6, 18.1.8, and 19.1.6 have since also been released (with 19.1.7 due to be released soon?). We tried to build each of those versions, but we were only successful in building versions 15.0.7 and 16.0.6, hence this issue.

When it comes to versions 17.0.6 and later, we were able to build everything but the debug version on Windows ARM (see llvm/llvm-project#88368). We tried to build LLVM+Clang with Clang itself, but then we came across an issue with duplicate rules to build clang.lib (see llvm/llvm-project#74694).

In the end, we want to keep things "simple", i.e. build libOpenCOR using MSVC on Windows, GCC on Linux, and Clang on macOS. So, it looks like we might be stuck with version 16.0.6...?