[ORC] Further improvements to SymbolStringPtr & NonOwningSymbolStringPtr.

A follow-up to https://reviews.llvm.org/D142314:

* Make SymbolStringPtrs constructible from NonOwningSymbolStringPtrs.

* Move and rename getRefCount and isValid (now poolEntryIsAlive) to improve
readability. Also updates these routines to make them safe for use with
sentinel values (null, empty, tombstone).

* Move ref-counting operations into their own incRef and decRef methods.