[LLVM] Update C++ standard to 17

Also make the soft toolchain requirements hard. This allows
us to use C++17 features in LLVM now.

If we find patterns with C++17 that improve readability
it should be recommended in the coding standards.

Reviewed By: jhenderson, cor3ntin, MaskRay

Differential Revision: https://reviews.llvm.org/D130689