ObjectFileELF: Fix symbol lookup in bss section

Summary:
If we have symbol information in a separate file, we need to be very
careful about presenting a unified section view of module to the rest of
the debugger. ObjectFileELF had code to handle that, but it was being
overly cautious -- the section->GetFileSize()!=0 meant that the
unification would fail for sections which do not occupy any space in the
object file (e.g., .bss). In my case, that manifested itself as not
being able to display the values of .bss variables properly as the
section associated with the variable did not have it's load address set
(because it was not present in the unified section list).

I test this by making sure the unified section list and the variables
refer to the same section.

Reviewers: eugene, zturner

Subscribers: tberghammer, lldb-commits, mgorny

Differential Revision: https://reviews.llvm.org/D32434

llvm-svn: 301917