Replace MergeOutputSection with a synthetic section.

With a synthetic merge section we can have, for example, a single
.rodata section with stings, fixed sized constants and non merge
constants.

I can be simplified further by not setting Entsize, but that is
probably better done is a followup patch.

This should allow some cleanup in the linker script code now that
every output section command maps to just one output section.

llvm-svn: 294005