[ORC] Add a public unsafe-operations helper for SymbolStringPtr.

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.