[clang] [Clang][Driver] Skip empty strings in getAArch64MultilibFlags (PR #97827)

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asmok-g wrote:

@statham-arm you're right. I need to verify this is the real culprit as I didn't fully build the toolchain at the previous revision. I used a fast build instead, so I need to verify..

@smithp35 it is actually `-Wc++20-compat-aggregate-init`.. with an internal prefix IIUC. Not so sure I fully understand the warning structure.

https://github.com/llvm/llvm-project/pull/97827