[clang] [HIP][SPIRV] Enable the SPIRV backend instead of the translator through an experimental flag. (PR #162282)

[Shilei Tian via cfe-commits]

================
@@ -1421,6 +1421,18 @@ def fhip_emit_relocatable : Flag<["-"], "fhip-emit-relocatable">,
   HelpText<"Compile HIP source to relocatable">;
 def fno_hip_emit_relocatable : Flag<["-"], "fno-hip-emit-relocatable">,
   HelpText<"Do not override toolchain to compile HIP source to relocatable">;
+def hip_use_experimental_spirv_backend
----------------
shiltian wrote:

Maybe just `use-experimental-spirv-backend` and it can also be used by other targets.

https://github.com/llvm/llvm-project/pull/162282