llvm-16 requires C++17

[anbe42]

but CMakeLists.txt still only wants CMAKE_CXX_STANDARD 14, resulting in compilation with -std=gnu++14 which results in
.../include/llvm/ADT/bit.h:64:56: error: ‘is_integral_v’ is not a member of ‘std’

see also KhronosGroup/SPIRV-LLVM-Translator@75d16c2

[frantisekz]

This is trivially fixed by #416 .