[clang] [AIX][TOC] -mtocdata/-mno-tocdata fix non deterministic iteration order (PR #86840)

[Sean Fertile via cfe-commits]

https://github.com/mandlebug commented:

It's unfortunate we have to ditch the StringSet because of the iteration order since the semantics of the option are the same regardless of the order the names are printed in. 

Now that we are constructing the list in sorted order we should change [these](https://github.com/llvm/llvm-project/blob/36e74cfdbde208e384c72bcb52ea638303fb7d67/clang/lib/Frontend/CompilerInstance.cpp#L1052) to assertions that they are already sorted.

https://github.com/llvm/llvm-project/pull/86840