Fix dangling reference in DwarfLinker.cpp. The original code

  Seq.emplace_back(Seq.back());

does not work as planned, since Seq.back() may become a dangling reference
when emplace_back is called and possibly reallocates vector. To avoid this,
the vector allocation should be reserved first and only then used.

This broke test/tools/dsymutil/X86/custom-line-table.test with Visual C++ 2013.

llvm-svn: 244405