Use the section name if a STT_SECTION symbol has empty name.

Without this we would have multiple relocations pointing to symbols
with the same name: the empty string. There was no way for yaml2obj to
be able to handle that.

A more general solution would be to unique symbol names in a similar
way to how we unique section names.  In practice I think this covers
all common cases and is a bit more user friendly than using names like
sym1, sym2, sym3, etc.

llvm-svn: 312603