[clang] Revert "[Clang][AMDGPU] Change __fp16 to _Float16 in builtin definitions" (PR #185861)

[Shilei Tian via cfe-commits]

shiltian wrote:

Is it possible to trigger a downstream all-component build with upstream LLVM to catch all build failures? The root cause seems to be downstream code relying on implicit type conversions for a long time, which is essentially incorrect, IMHO. The recent change from `_Vector` to `_ExtVector` exposed the issue, and the change from `__fp16` to `_Float16` is just a symptom, not the root cause.

CC @jhuber6 @kzhuravl 

https://github.com/llvm/llvm-project/pull/185861