[llvm-objcopy] Make modifications in-place if output is not specified

If the output file is not specified make the modifications in-place 
(like binutils objcopy does). In particular, this fixes 
the behavior of Clang -gsplit-dwarf (if Clang is configured to use llvm-objcopy), 
previously it was creating .dwo files, but still leaving *dwo* sections in 
the original binary.

Test plan: make check-all

Differential revision: https://reviews.llvm.org/D42873

llvm-svn: 324783