Special case the creation of relocation sections.

Special case the creation of relocation sections.

These sections are never looked up and we know when have to create them. Use
that to save adding them to the regular map and avoid a symbol->string->symbol
conversion for the group symbol.

This also makes the implementation independent of the details of how unique
sections are implemented.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@233539 91177308-0d34-0410-b5e6-96231b3b80d8