[XLA:GPU] Fix logic for symbol lookup
[XLA:GPU] Fix logic for symbol lookup

Currently, we return/log bad Status when symbol is not found, but the logic for
resolving the constants does not expect it to be always found. This leads to
two issues:

1. Log spam from "Failed to found symbol" error messages.
2. Hiding actual errors, if the symbol failed to resolve in some other ways.

Returning StatusOr<optional<>> achieves both goals.
