[clang] [Clang] Use valid source loc for empty target_clones diagnostic (PR #173747)

[via cfe-commits]

https://github.com/Sirraide commented:

I don’t know much about this attribute, but this seems reasonable, though it’s missing a release note.

> I am not sure whether this is intentional, but for AArch64 the validation does not emit a diagnostic for missing "default" argument.

Yeah, the ARM version apparently doesn’t attempt to access the location of the first parameter, so I think it’s fine to leave that as-is.

https://github.com/llvm/llvm-project/pull/173747