[ORC] Add a NonOwningSymbolStringPtr utility.

Introduces a non-owning SymbolStringPool entry pointer. Instances of the new
type can be compared with SymbolStringPtr instances, but do not participate in
ref-counting and are therefore cheaper to copy. This makes it efficient to use
in algorithms that use symbol-strings as ids, e.g. ORC's waiting-on graph. A
future commit will rewrite ORC's waiting-on graph.

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D142314