[ELF] - Stop naming relocation sections with first input section name.

It was reported (https://reviews.llvm.org/D38724#902841) that when we use 
-ffunction-sections --emit-relocs build, REL[A] output section receives the name of first
input section, like .rela.text.first_function_in_text rather than .rela.text.

It is probably not really an issue as sh_info still points to correct target section, but
it does not look clean in output and allows internal section name to leak there,
what at least looks confusing and is not consistent with ld.bfd.

Patch changes this behavior so that target output section name is used as a base.

Differential revision: https://reviews.llvm.org/D39242

llvm-svn: 316760