[Clang][Driver] recheck for nullptr

[nickdesaulniers]

Summary

This was flagged in https://www.viva64.com/en/b/0629/ under "Snippet No.
47".

Looks like TC is checked for nullptr once, but not the second time.
Fixes r275645.

[RKSimon]

According to the coverage report we don't have any coverage of TC == null at all - not sure if its (a) possible to add tests or (b) if we can ever have a non-null TC - so should we just assert?

http://lab.llvm.org:8080/coverage/coverage-reports/clang/coverage/Users/buildslave/jenkins/workspace/clang-stage2-coverage-R/llvm/tools/clang/lib/Driver/Driver.cpp.html

clang/lib/Driver/Driver.cpp

OA->doOnEachDependence(
           if (!IsFirst)
             os << ", ";
           os << '"';
           if (TC)
             os << A->getOffloadingKindPrefix();
           else
             os << "host";
           os << " (";
+          if (TC)
             os << TC->getTriple().normalize();
 
           if (BoundArch)
             os << ":" << BoundArch;
           os << ")";
           os << '"';
           os << " {" << PrintActions1(C, A, Ids) << "}";
           IsFirst = false;
         });