Fix the undefined-but-used odr-use marker (DR48)

We should treat tentative definitions as undefined for the purpose of
ODR-use linkage checking.

This broke somewhere around r149731 when tests were disabled.

Note that test coverage for these diagnostics is generally lacking due to a
separate issue (PR19910: Don't suppress unused/undefined warnings when there
are errors).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@209996 91177308-0d34-0410-b5e6-96231b3b80d8