Fix what appears to be a copy-paste error.

This looks like it was copied from SetUpBuildDumpLog, which dumps to the
file `DiagOpts->DumpBuildInformation`. There is another member
`DiagOpts->DiagnosticLogFile` which appears to be unused. The fact that
this feature doesn't even print to the output file specified on the
command line makes me think that it should be ripped out.

llvm-svn: 172944