IWYU: fix 0.14 build (#16761)
IWYU: fix 0.14 build (#16761)

* IWYU: fix 0.14 build

The CMake patch used for 0.13 hadn't been applied to the master when
0.14 was released, and this version of IWYU requires C++14 or higher.

* Flake8
LLNL/spack,LLNL/spack,iulian787/spack,LLNL/spack,iulian787/spack,iulian787/spack,iulian787/spack,iulian787/spack,LLNL/spack,LLNL/spack