[clang] [OpenCL] Add decls for cl_intel_bfloat16_conversions (PR #150393)

[Mészáros Gergely via cfe-commits]

Maetveis wrote:

FYI: See the llvm SPIR-V backend doing the right thing if given a declaration on compiler explorer: https://godbolt.org/z/ac1ThYePG

https://github.com/llvm/llvm-project/pull/150393