Debug Info: Fix for break due to r181271

Apparently we didn't keep an association of Compile Unit metadata nodes
to DIEs so looking up that parental context failed & thus caused no
DW_TAG_imported_modules to be emitted at the CU scope. Fix this by
adding the mapping & sure up the test case to verify this.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181339 91177308-0d34-0410-b5e6-96231b3b80d8