Introduce AutoDiffIndexSubset

This PR overhauls data structures used for parameter indices in the AutoDiff infrastructure in SIL.

Previously, we used llvm::SmallBitVector to represent differentiation parameter indices in both AST and SIL. It was not efficient, and most importantly there's no way to put this in an instruction without causing memory leaks.

This change replaces all uses of llvm::SmallBitVector in SIL AutoDiff code paths with a ASTContext-uniqued AutoDiffIndexSubset * where bits are stored as trailing objects.

AutoDiffIndexSubset does not have "parameter indices" in its name because it is not only designed for parameter indices, but also for result indices as we move to supporting multi-result differentiation. AutoDiffIndexSubset has set operations like isSubsetOf, isSupersetOf, and contains, but it also has a special capacity property. All differentiable function's parameter indices data should store the number of parameters as capacity, so that the differentiation transform won't need special logic to check whether an index is out of range.

Another minor change is the module format layout of SILDifferentiableAttr. It used to store parameter indices as consecutive bool bits, but now stores numeric parameter indices directly for efficiency.

It will be necessary to refactor or eliminate AutoDiffParameterIndices to make use of AutoDiffIndexSubset. AutoDiffParameterIndices is at the AST level, so it is not in the scope for this PR.

Unblocks #23482. Partially resolves TF-67.