[lld-macho][NFC] Minor refactor of Writer::run()

Move some functions closer to their uses. Move detailed address-assignment logic out of the otherwise abstract `Writer::run()`. This prepares the ground for a diff to implement branch range extension thunks.

* `SyntheticSections.cpp`
 ** move `needsBinding()` and `prepareBranchTarget()` into `Writer.cpp`
 ** move `addNonLazyBindingEntries()` adjacent to its use.

* `Writer.cpp`
 ** move address-assignment logic from `Writer::run()` into new function `Writer::assignAddresses()`
 ** move `needsBinding()` and `prepareBranchTarget()` from `SyntheticSections.cpp`

* `Target.h`
** remove orphaned decls of `prepareSymbolRelocation()` and `validateRelocationInfo()` which were moved to other files in earlier diffs.

Differential Revision: https://reviews.llvm.org/D98795