This commit switches the option to `-std=c++14` to avoid potential ODR
violations and fix the Mac build.
This commit downgrades from `-std=c++17` to `-std=c++14` because
"llvm/Support/RWMutex.h" from clang-r370808 will use `std::shared_mutex`
when `-std=c++17` is specified but LLVM/Clang prebuilts are built with a
different implementation.
Furthermore, Mac OS X 10.8 (or 10.9) does not support
`std::shared_mutex`. Thus, there are availability errors when we are
building header-checker for Mac OS X:
clang-r370808/include/llvm/Support/RWMutex.h:98:8:
error: 'shared_mutex' is unavailable: introduced in macOS 10.12
Bug: 139945549
Test: OUT_DIR=out && \
development/vndk/tools/header-checker/android/build-prebuilts.sh
Change-Id: Ic23fbdb67f7cd0700a3001c644485f7d8d0ee3e2
