[clang] let Neon builtin function accept a const variable (PR #144625)

[via cfe-commits]

scout-zeng wrote:

Hi @efriedma-quic , I do not think this workaround can fix this issue. Because the clang compiler still gives me an error like 'argument to xxx must be a constant integer", which means the if condition in line 5670 is not true. I am confused about that. Can you give me some suggestions ?

https://github.com/llvm/llvm-project/pull/144625