Debug info: use the file a typedef is defined in as its decl_file instead
of the current compilation unit.
As a side effect this enables many more LTO uniquing opportunities.

This reapplies r199757 with a better testcase.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199760 91177308-0d34-0410-b5e6-96231b3b80d8