DebugInfo: Support namespace aliases as DW_TAG_imported_declaration instead of DW_TAG_imported_module

I really should read the spec more often (and test GCC more often too).
I just assumed that namespace aliases would be the same as using
directives, except with a name. But apparently that's not how the DWARF
standards suggests they be implemented. DWARF4 provides an example and
other non-normative text suggesting that namespace aliases be
implemented by named imported declarations intsead of named imported
modules.

So be it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205685 91177308-0d34-0410-b5e6-96231b3b80d8