PR14606: debug info imported_module support

Adding another CU-wide list, in this case of imported_modules (since they
should be relatively rare, it seemed better to add a list where each element
had a "context" value, rather than add a (usually empty) list to every scope).
This takes care of DW_TAG_imported_module, but to fully address PR14606 we'll
need to expand this to cover DW_TAG_imported_declaration too.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179836 91177308-0d34-0410-b5e6-96231b3b80d8