[llvm-mc] - Do not crash when referencing undefined debug sections.

MC has code that pre-creates few debug sections:
https://github.com/llvm-mirror/llvm/blob/master/lib/MC/MCObjectFileInfo.cpp#L396

If users code has a reference to such section but does not redefine it,
MC code currently asserts, because still thinks they are normally defined.

The patch fixes the issue.

Differential revision: https://reviews.llvm.org/D55173

llvm-svn: 348243