[clang][aarch64] Fix mangling of bfloat16 neon vectors

The AAPCS64 specifies the internal type is used for c++ mangling. For
bfloat16 it was defined as `BFloat16` when it should be `Bfloat16`, i.e.
lowercase 'f'.

For more information, see:

https://github.com/ARM-software/abi-aa/blob/master/aapcs64/aapcs64.rst#appendix-support-for-advanced-simd-extensions

Reviewed By: stuij

Differential Revision: https://reviews.llvm.org/D87463