Fix null pointer access for discriminator assignment.

[danielcdh]

Summary

This fixes the buildbot failure.

llvm/trunk/lib/Transforms/Utils/AddDiscriminators.cpp

bool AddDiscriminators::runOnFunction(Function &F) {
   for (BasicBlock &B : F) {
     LocationSet CallLocations;
     for (auto &I : B.getInstList()) {
       CallInst *Current = dyn_cast<CallInst>(&I);
       if (!Current || isa<DbgInfoIntrinsic>(&I))
         continue;
 
       DILocation *CurrentDIL = Current->getDebugLoc();
+      if (!CurrentDIL)
+        continue;
       Location L =
           std::make_pair(CurrentDIL->getFilename(), CurrentDIL->getLine());
       if (!CallLocations.insert(L).second) {
         auto *Scope = CurrentDIL->getScope();
         auto *File = Builder.createFile(CurrentDIL->getFilename(),
                                         Scope->getDirectory());
         auto *NewScope = Builder.createLexicalBlockFile(Scope, File, ++LDM[L]);
         Current->setDebugLoc(DILocation::get(Ctx, CurrentDIL->getLine(),
                                              CurrentDIL->getColumn(), NewScope,
                                              CurrentDIL->getInlinedAt()));
         Changed = true;
       }
     }
   }
   return Changed;
 }