[lld] [lld]Add lld/Common/BPSectionOrdererBase from MachO for reuse in ELF (PR #117514)

[Fangrui Song via llvm-commits]

MaskRay wrote:

Spent more time on BP. I believe that the virtual classes as done by this PR `BPSectionBase`/`BPSymbol` is quite wasteful.   `BPSectionBase`/`BPSymbol`  are thin wrappers that contain just one pointer. This is quite wasteful.

Perhaps we should treat Symbol and InputSection as `void *` in BPSectionOrderer.cpp and use the following class hierarchy instead.

```
class BPBase {
  virtual getSectionSize(void *);
  virtual isCodeSection(void *);
  
  virtual StringRef getSymbolName(void *);
  virtual uint64_t getSymbolValue(void *);
};
class BPMachO;
class BPELF; // future
```

Then we can avoid allocating a large section and `inputSections` (expensive for large ELF applications).

We could also leverage Curiously Recurring Template Pattern if we want to retain type information of symbols and sections.

https://github.com/llvm/llvm-project/pull/117514