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

[David Tarditi via cfe-commits]

================

----------------
dtarditi wrote:

I agree that it is fine to not handle this for now. 
 
If we do decide to handle this, we  need to be careful because this equivalence does not hold for signed integer arithmetic expressions in C.  The equivalence relies on commutativity and associativity of integer arithmetic. Reassociation can change whether overflow occurs for integer expressions at runtime and signed integer overflow is undefined behavior in C.  If, however, we assume that signed integer arithmetic is 2's complement arithmetic (which can be specified via a compiler flag), then this equivalence holds for signed integer expressions.

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