This patch prefers cxx17 to cxx14, if it's available.
Please let me know if there's a good reason to stay at cxx14-only. The
rest of llvm seems to be using cxx17 without problems, and I'd like to
use the 'static inline' members feature of cxx17 to solve a GWP-ASan
dynamic initialization problem in Scudo standalone.
Actually I see c++14 still there
Why we don't use?
set (CMAKE_CXX_STANDARD 17)