[clang] [clang] Convert second arg of __builtin_assume_aligned to ConstantExpr (PR #161314)

[Timm Baeder via cfe-commits]

tbaederr wrote:

It seems like we're doing the load in `VisitConstantExpr` manually, even though that's already happened in this case, because that's what we do in `evaluateAsRValue`.

https://github.com/llvm/llvm-project/pull/161314