[Builtins] Add missing the macro 'y' description in comments

New type str 'y' added from D76077. But missed its description in
comments.

Reviewed By: stuij

Differential Revision: https://reviews.llvm.org/D116509

Author: tclin914

clang/include/clang/Basic/Builtins.def

@@ -26,6 +26,7 @@
 //  i -> int
 //  h -> half (__fp16, OpenCL)
 //  x -> half (_Float16)
+//  y -> half (__bf16)
 //  f -> float
 //  d -> double
 //  z -> size_t