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

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Sirraide wrote:

Ok, I’ve added some more tests involving template instantiation (and also implemented actually handling `assume` in TreeTransform because I seem to have forgotten about that earlier...); just let me know if there are any other specific situations that we should test.

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