In this mode it just tries to tail merge the strings without imposing any other
format constrains. It will not, for example, add a null byte between them.
Also add support for keeping a tentative size and offset if we decide to
not optimize after all.
This will be used shortly in lld for merging SHF_STRINGS sections.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@251153 91177308-0d34-0410-b5e6-96231b3b80d8