DebugInfo: Don't call DIBuilder::retainType(nullptr)

An upcoming LLVM commit will make calling
`DIBuilder::retainType(nullptr)` illegal (actually, it already was, but
it wasn't verified).  Check for null before calling.

This triggered in test/CodeGenObjC/debug-info-block-helper.m.

llvm-svn: 233443

Author: dexonsmith

clang/lib/CodeGen/CGDebugInfo.cpp

@@ -3432,7 +3432,8 @@ void CGDebugInfo::finalize() {
 void CGDebugInfo::EmitExplicitCastType(QualType Ty) {
   if (CGM.getCodeGenOpts().getDebugInfo() < CodeGenOptions::LimitedDebugInfo)
     return;
-  llvm::DIType DieTy = getOrCreateType(Ty, getOrCreateMainFile());
-  // Don't ignore in case of explicit cast where it is referenced indirectly.
-  DBuilder.retainType(DieTy);
+
+  if (llvm::DIType DieTy = getOrCreateType(Ty, getOrCreateMainFile()))
+    // Don't ignore in case of explicit cast where it is referenced indirectly.
+    DBuilder.retainType(DieTy);
 }