[ORC-RT] Add a SymbolStringPool class to the ORC runtime.

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).

GitOrigin-RevId: 8fda8901e29bf80bd91574f29b50868958195a4e
3 files changed