[lld-macho][NFC] when reasonable, replace auto keyword with type names

lld policy discourages `auto`. Replace it with a type name whenever reasonable. Retain `auto` to avoid ...
* redundancy, as for decls such as `auto *t = mumble_cast<TYPE *>` or similar that specifies the result type on the RHS
* verbosity, as for iterators
* gratuitous suffering, as for lambdas

Along the way, add `const` when appropriate.

Note: a future diff will ...
* add more `const` qualifiers
* remove `opt::` when we are already `using llvm::opt`

Differential Revision: https://reviews.llvm.org/D98313