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

[Peter Smith via cfe-commits]

smithp35 wrote:

I can't find a `-Wno-something-c++20-aggregate-init` in clang. 

The closest I can find is https://clang.llvm.org/docs/DiagnosticsReference.html#wc-20-compat which has a text string
```
warning: aggregate initialization of type A with user-declared constructors is incompatible with C++20
```
but this doesn't seem to have an individual warning. Did you mean to use `-Wc++20-compat`?

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