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

This reverts commit r181393 (git 3923d6a87fe7b2c91cc4a7dbd90c4ec7e2316bcd).

This seems to be emitting too much extra debug info for two (known)
reasons:
* full class definitions are emitted when only declarations are expected
* unused using declarations still produce DW_TAG_imported_declarations