Use the unified section list when generating a symbol table

Summary:
Symbol table generation code was failing to take into account the debug symbols because
the object file was looking only into its own section list when doing the generation, even though
the debug symbols from another object file were correctly detected and loaded by the
SymbolVendor. This changes the code to use the unified section list, which fixes this problem.

Test Plan:
I do not intend do submit this yet since it causes (or more like, exposes) the issue
in D7884, but I wanted to put this out here, so that anyone who wants to take a look at it can do
so. (And I also wanted to know if this is the right approach to the problem :).

Reviewers: clayborg, zturner

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D7913

llvm-svn: 231229