Fix crash in DebugInfoFinder when adding a module with forward declared composite type

The testcase that is included in the patch caused a crash when doing DebugInfoFinder::processModule
on the module due to DCT->getElements() returning nullptr in DebugInfoFinder::processType.

By doing "DCT->getElements()" instead of "DCT->getElements()->operands()" one gets a DIArray
instead of a raw MDTuple. The former has code to handle null as a 0-element array and
therefore avoids the crash.

Differential Revision: http://reviews.llvm.org/D9008

llvm-svn: 234875