[lldb] Fix evaluation of nested classes with parent from other CU

This makes sure that we associate DIEs that are imported from other CUs with the appropriate decl context.

Without this fix, nested classes can be dumped directly into their CU context if their parent was imported from another CU.

Reviewed By: teemperor, labath

Differential Revision: https://reviews.llvm.org/D68278

Patch by Jaroslav Sevcik!

llvm-svn: 373470