Debug Info: Using declarations/DW_TAG_imported_declaration of variables, types, and functions.

Basic support is implemented here - it still doesn't account for
declared-but-not-defined variables or functions. It cannot handle out of
order (declared, 'using', then defined) cases for variables, but can
handle that for functions (& can handle declared, 'using'd, and not
defined at all cases for types).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@181393 91177308-0d34-0410-b5e6-96231b3b80d8