[ORC] Tailor ELF debugger support plugin to load-address patching only (#168518)

In 4 years the ELF debugger support plugin wasn't adapted to other
object formats or debugging approaches. After the renaming NFC in
https://github.com/llvm/llvm-project/pull/168343, this patch tailors the
plugin to ELF and section load-address patching. It allows removal of
abstractions and consolidate processing steps with the newly enabled
AllocActions from https://github.com/llvm/llvm-project/pull/168343.

The key change is to process debug sections in one place in a
post-allocation pass. Since we can handle the endianness of the ELF file
the single `visitSectionLoadAddresses()` visitor function now, we don't
need to track debug objects and sections in template classes anymore. We
keep using the `DebugObject` class and drop `DebugObjectSection`,
`ELFDebugObjectSection<ELFT>` and `ELFDebugObject`.

Furthermore, we now use the allocation's working memory for load-address
fixups directly. We can drop the `WritableMemoryBuffer` from the debug
object and most of the `finalizeWorkingMemory()` step, which saves one
copy of the entire debug object buffer. Inlining `finalizeAsync()` into
the pre-fixup pass simplifies quite some logic.

We still track `RegisteredObjs` here, because we want to free memory
once the corresponding code is freed. There will be a follow-up patch
that turns it into a dealloc action.

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