DebugInfo: Partial revert r209984 due to more cases where abstract DbgVariables do not have associated DIEs.

Along with a test case to demonstrate that due to inlining order there
are cases where abstract variable DIEs are not constructed since the
abstract subprogram was built due to a previous inlining that optimized
away those variables. This produces incorrect debug info (the 'missing'
abstract variable causes the inlined instance of that variable to be
emitted with a full description (name, line, file) rather than
referencing the abstract origin), but this commit at least ensures that
it doesn't crash...

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210143 91177308-0d34-0410-b5e6-96231b3b80d8