[JITLink] Pass Allocator (rather than storage) into Symbol named constructors.

Also switch from orc::ExecutorAddrDiff to uint64_t for the Symbol::Size field.

These changes help to prepare for the introduction of symbol alias support:
Aliases will require an auxiliary data structure which will also need to be
allocated (hence the need to pass the allocator down). The Size field will be
re-tasked to track the auxiliary data (which will hold a replacement Size field)
if the symbol is either an alias, or aliased by some other symbol.