[ORC] Introduce lookupAndRecordAddrs utility.

Accepts a vector of (SymbolStringPtr, ExecutorAddress*) pairs, looks up all the
symbols, then writes their address to each of the corresponding
ExecutorAddresses.

This idiom (looking up and recording addresses into a specific set of variables)
is used in MachOPlatform and the (temporarily reverted) ELFNixPlatform, and is
likely to be used in other places in the near future, so wrapping it in a
utility function should save us some boilerplate.