Revert r184401 which reverted r183462.

The problem with r183462 was that we assumed that a diagnostic id of
zero would be silent.

This small correction to CheckDerivedToBaseConversion changes it's
behavior to omit the diagnostic when given a diagnostic id of zero.

This fix passes the test case added in r184402.


git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@184631 91177308-0d34-0410-b5e6-96231b3b80d8