[clang] [llvm] [mlir] Treewide C++26/2d fixes (PR #223284)

[Nikita Popov via llvm-commits]

nikic wrote:

Can you please split this into smaller parts? This mixes lots of unrelated issues in different parts of the codebase, and some of these will be easier to reach consensus on than others.

At the very least, the parts doing some kind of non-mechanical change (like the ELF and Clang CodeGen changes you cite) should be separated out.

https://github.com/llvm/llvm-project/pull/223284