[clang] [Clang] Introduce malloc_span attribute (PR #167010)

[Aleksandr Nogikh via cfe-commits]

a-nogikh wrote:

This PR is a reworked version of https://github.com/llvm/llvm-project/pull/165433. The original PR proposed the changes to the semantics of the `malloc` attribute itself, but the consensus was that adding a separate `malloc_attribute` would be a better approach.

https://github.com/llvm/llvm-project/pull/167010