[ELF] - Do not change binding of symbols when creating relocatable output.

Spec says "A hidden symbol contained in a relocatable object must be either
removed or converted to STB_LOCAL binding by the link-editor when the
relocatable object is included in an executable file or shared object".
But we previously converted symbols to STB_LOCAL even when -r was specified.

Broken binary was produced, this is PR28967, patch fixes the issue.

Differential revision: https://reviews.llvm.org/D23514

llvm-svn: 279220