Verifier: Add check for DICompositeType elements being null

Came up recently with some nodebug case on codeview, that caused a null
entry in elements and crashed LLVM.

Original clang fix to avoid generating IR like this: 504dd57

Author: dwblaikie

llvm/lib/IR/Verifier.cpp

@@ -1319,6 +1319,8 @@ void Verifier::visitDICompositeType(const DICompositeType &N) {
   unsigned DIBlockByRefStruct = 1 << 4;
   CheckDI((N.getFlags() & DIBlockByRefStruct) == 0,
           "DIBlockByRefStruct on DICompositeType is no longer supported", &N);
+  CheckDI(llvm::all_of(N.getElements(), [](const DINode *N) { return N; }),
+          "DISubprogram contains null entry in `elements` field", &N);
 
   if (N.isVector()) {
     const DINodeArray Elements = N.getElements();

llvm/test/Verifier/dicompositetype-elements-null.ll

@@ -0,0 +1,6 @@
+; RUN: not opt -S < %s 2>&1 | FileCheck %s
+
+!named = !{!0}
+; CHECK: DISubprogram contains null entry in `elements` field
+!0 = distinct !DICompositeType(tag: DW_TAG_structure_type, name: "t1", elements: !1)
+!1 = !{null}