[Clang] Fix the normalization of fold constraints (#177531) Fold constraints can contain packs expanded from different locations. For `C<Ps...>`, where the ellipsis immediately follows the argument, the pack should be expanded in place regardless of the fold expression. For `C<Ps> && ...`, the fold expression itself is responsible for expanding Ps. Previously, both kinds of packs were expanded by the fold expression, which broke assumptions within concept caching. This patch fixes that by preserving PackExpansionTypes for the first kind of pack while rewriting them to non-packs for the second kind. This patch also removes an unused function and performs some cleanup of the evaluation contexts. Hopefully it is viable for backporting. No release note, as this issue was a regression. Fixes https://github.com/llvm/llvm-project/issues/177245
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