[clang][BFloat] Avoid redefining bfloat16_t in arm_neon.h

As of https://reviews.llvm.org/D79708, clang-tblgen generates `arm_neon.h`,
`arm_sve.h` and `arm_bf16.h`, and all those generated files will contain a
typedef of `bfloat16_t`. However, `arm_neon.h` and `arm_sve.h` include
`arm_bf16.h` immediately before their own typedef:

    #include <arm_bf16.h>
    typedef __bf16 bfloat16_t;

With a recent version of clang (I used 16.0.1) this results in warnings:

    /usr/lib/clang/16/include/arm_neon.h:38:16: error: redefinition of typedef 'bfloat16_t' is a C11 feature [-Werror,-Wtypedef-redefinition]

Since `arm_bf16.h` is very likely supposed to be the one true place where
`bfloat16_t` is defined, I propose to delete the duplicate typedefs from the
generated `arm_neon.h` and `arm_sve.h`.

Reviewed By: sdesmalen, simonbutcher

Differential Revision: https://reviews.llvm.org/D148822