We should use `orc_rt` as the public C++ API namespace for the ORC runtime and
control symbol visibility to hide implementation details, rather than rely on
the '__' prefix.
This is a counterpart to llvm::orc::SymbolStringPool. It holds uniqued,
ref-counted strings; and can be used to avoid redundant storage of strings,
and speed up comparison of strings held in the pool (these become pointer
comparisons).