Revert "Debug Info: Using declarations/DW_TAG_imported_declaration of...

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

This reverts commit r181393 (git 3923d6a8).

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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@181947 91177308-0d34-0410-b5e6-96231b3b80d8