[ARM,CDE] Improve CDE intrinsics testing

Summary:
This patch:
* adds tests for vreinterpret intinsics in big-endian mode
* adds C++ runs to the CDE+MVE header compatibility test

Reviewers: simon_tatham, MarkMurrayARM, ostannard, dmgreen

Reviewed By: simon_tatham

Subscribers: kristof.beyls, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D76927