Allow DwarfCompileUnit::constructImportedEntityDIE to instanciate a GlobalVariable DIE.

Usually global variables are in a retain list and instanciated before
any call to constructImportedEntityDIE is made. This isn't true for
forward declarations though.
The testcase for this change is generated by a clang patched to emit
such forward declarations (patch at http://reviews.llvm.org/D6173
which will land soon). The updated testcase tests more than just
global variables, it now tests every type of 'using' clause we
support.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222217 91177308-0d34-0410-b5e6-96231b3b80d8