[llvm-objcopy] Rename relocation sections together with their targets.

As for now, llvm-objcopy renames only sections that are specified
explicitly in --rename-section, while GNU objcopy keeps names of
relocation sections in sync with their targets. For example:

> readelf -S test.o
...
  [ 1] .foo      PROGBITS
  [ 2] .rela.foo RELA

> objcopy --rename-section .foo=.bar test.o gnu.o
> readelf -S gnu.o
...
  [ 1] .bar      PROGBITS
  [ 2] .rela.bar RELA

> llvm-objcopy --rename-section .foo=.bar test.o llvm.o
> readelf -S llvm.o
...
  [ 1] .bar      PROGBITS
  [ 2] .rela.foo RELA

This patch makes llvm-objcopy to match the behavior of GNU objcopy better.

Differential Revision: https://reviews.llvm.org/D110352