[objcopy][NFC] Add doc comments to the executeObjcopy* functions.

Add doc comments to the executeObjcopy* functions.

Depends on D88827

[llvm-objcopy][NFC] Move core implementation of llvm-objcopy into separate library.

This patch moves core implementation of llvm-objcopy into Object library
(http://lists.llvm.org/pipermail/llvm-dev

[llvm-objcopy][NFC] Move core implementation of llvm-objcopy into separate library.

This patch moves core implementation of llvm-objcopy into Object library
(http://lists.llvm.org/pipermail/llvm-dev/2020-September/145075.html).
The functionality for parsing input options is left inside tools/llvm-objcopy.
The interface of ObjCopy library:

ObjCopy/ELF/ELFObjcopy.h

```
Error executeObjcopyOnIHex(const CopyConfig &Config, MemoryBuffer &In,
Buffer &Out);
Error executeObjcopyOnRawBinary(const CopyConfig &Config, MemoryBuffer &In,
Buffer &Out);
Error executeObjcopyOnBinary(const CopyConfig &Config,
object::ELFObjectFileBase &In, Buffer &Out);

```
ObjCopy/COFF/COFFObjcopy.h

```
Error executeObjcopyOnBinary(const CopyConfig &Config,
object::COFFObjectFile &In, Buffer &Out);

```
ObjCopy/MachO/MachOObjcopy.h

```
Error executeObjcopyOnBinary(const CopyConfig &Config,
object::MachOObjectFile &In, Buffer &Out);

```
ObjCopy/wasm/WasmObjcopy.h

```
Error executeObjcopyOnBinary(const CopyConfig &Config,
object::WasmObjectFile &In, Buffer &Out);

```

Differential Revision: https://reviews.llvm.org/D88827

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