This is a follow-up from https://reviews.llvm.org/D61717. Where Richard described the issue with compiling arm_neon.h under -flax-vector-conversions=none. It looks like the example reproducer does actually work:
echo '#include <arm_neon.h>' | clang -target arm64-linux-gnu -arch +neon -fsyntax-only -x c - -fno-lax-vector-conversions
but was missing the -ffreestanding compiler option otherwise the compiler would select the system headers from /usr. However, arm-neon-header.c test was missing this target triple 'arm64-linux-gnu' so I've added it here.