[clang] [-Wunsafe-buffer-usage] Add alloc_size knowledge to the 2-param span constructor warning (PR #114894)

[Malavika Samak via cfe-commits]

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malavikasamak wrote:

Can you please rebase this PR? The isSafeSpanTwoParamConstruct matcher has changes on the main branch that are not present here. For instance, case 6 is now used by :`std::span<T>{std::addressof(...), 1}` on the main branch.

https://github.com/llvm/llvm-project/pull/114894