[clang] [Clang] Ignore assumptions with side effects at compile time (PR #85534)

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Sirraide wrote:

The standard also says that ‘The expression is not evaluated’ [[dcl.attr.assume]](https://eel.is/c++draft/dcl.attr.assume#sentence-3), so we can only really get away with evaluating it by the as-if rule iff it has no side-effects, so this is just bringing compile-time in line with what we’ve already decided to do in codegen.

https://github.com/llvm/llvm-project/pull/85534