Remangler: Implement the hash map for substitutions in the spirit of llvm's SmallPtrSet.
Remangler: Implement the hash map for substitutions in the spirit of llvm's SmallPtrSet.

This avoids malloc calls in the common case. Only if there are more than 16 substitutions in the fixed-sized array, the overflow goes into a std::unordered_map.

SR-10028
rdar://problem/48575729

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