[clang] [Clang][C++23] Implement P1774R8: Portable assumptions (PR #81014)

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Sirraide wrote:

> I don't see any tests validating that these are being properly instantiated (either by AST dump or by codegen), the only template cases i see are ones where the instantiation not happening doesn't 'matter'. Also, I don't see how these are affecting calls through concepts, so I'd like to see testing for that as well (that is, how the attribute with a failure during instantiation does).

I’ll add some more tests then. 


https://github.com/llvm/llvm-project/pull/81014