SymbolStringPoolEntryUnsafe provides unsafe access to SymbolStringPtr objects,
allowing clients to manually retain and release pool entries, or consume or
create SymbolStringPtr instances without affecting an entry's ref-count. This
can be useful when writing C APIs that need to handle SymbolStringPtrs.
As part of this patch the LLVM-C API implementation is updated to use the new
utility, rather than the old, private OrcV2CAPIHelper utility.
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