[llvm-objcopy] remove split dwo file creation from executeObjcopyOnBinary.

This patch removes creation of the resulting file from the
executeObjcopyOnBinary() function. For the most use cases, the
executeObjcopyOnBinary receives output file as a parameter
- raw_ostream &Out. The splitting .dwo file is implemented differently:
file containg .dwo tables is created inside executeObjcopyOnBinary().
When objcopy functionality would be moved into separate library,
current implementation will become inconvenient. The goal of that
refactoring is to separate concerns: It might be convenient to
to do dwo tables splitting but to create resulting file differently.

Differential Revision: https://reviews.llvm.org/D98582