Force SHF_MERGE optimizations with -r.

Some tools (dwarfdump for example) get confused by the current -O0 -r
output since it has multiple copies of .debug_str.

We cannot just merge sections with the same name as they can have
different sh_entsize.

We could have duplicated logic for merging sections based on name and
sh_entsize, but it seems better to just use the existing logic by
enabling optimizations.

llvm-svn: 328640