[clang] [clang][x86][bytecode] added trailing type for ffs, missed in pr llvm#162346 for llvm#160288 (PR #163905)

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donneypr wrote:

@RKSimon @tbaederr Following up on our discussion in #162346, I've created PR to add the trailing return type for the function that was missed. 

https://github.com/llvm/llvm-project/pull/163905