Android NDK r16b comes with

  Android clang version 5.0.300080  (based on LLVM 5.0.300080)

Although it claims version 5, it does not support the `-std=c++17`
or `-std=c++2a` flags that upstream Clang does.

Android NDK r17-beta1 comes with

  Android (4639204 based on r316199) clang version 6.0.1

that does have the flags.