[clang] [Headers] Create stub spirv64intrin.h (PR #131164)

[Jon Chesterfield via cfe-commits]

JonChesterfield wrote:

Clang raises a lot of exciting errors for spirv-- about vulcan environments and I don't really know the distinction between the two - if 32 bit spirv turns out to be a workable thing it should go down the same code path, with `ifdef SPIRV || SPIRV64` and a file rename. I don't know enough about the ecosystem to judge at this point.

https://github.com/llvm/llvm-project/pull/131164