[lld][Macho][NFC] Encapsulate priorities map in a priority class

`config->priorities` has been used to hold the intermediate state during the construction of the order in which sections should be laid out. This is not a good place to hold this state since the intermediate state is not a "configuration" for LLD. It should be encapsulated in a class for building a mapping from section to priority (which I created in this diff as the `PriorityBuilder` class).

The same thing is being done for `config->callGraphProfile`.

Reviewed By: #lld-macho, int3

Differential Revision: https://reviews.llvm.org/D122156