[TextAPI][elfabi] Make SoName optional

This change makes DT_SONAME treated as an optional trait for ELF TextAPI
stubs. This change accounts for the fact that shared objects aren't
guaranteed to have a DT_SONAME entry. Tests have been updated to check
for correct behavior of an optional soname.

Differential Revision: https://reviews.llvm.org/D55533

llvm-svn: 348817