Fix crash in DebugInfoFinder when adding a module with forward declared composite type.

[wanders]

Summary

The testcase that is included in the patch caused a crash when doing DebugInfoFinder::processModule on the module due to DCT->getElements() returning nullptr in DebugInfoFinder::processType.

By doing "DCT->getElements()" instead of "DCT->getElements()->operands()" one gets a DIArray instead of a raw MDTuple. The former has code to handle null as a 0-element array and therefore avoids the crash.

llvm/trunk/lib/IR/DebugInfo.cpp

void DebugInfoFinder::processType(DIType DT) {
   processScope(DT.getContext().resolve(TypeIdentifierMap));
   if (DICompositeType DCT = dyn_cast<MDCompositeTypeBase>(DT)) {
     processType(DCT.getTypeDerivedFrom().resolve(TypeIdentifierMap));
     if (DISubroutineType ST = dyn_cast<MDSubroutineType>(DCT)) {
       for (MDTypeRef Ref : ST->getTypeArray())
         processType(Ref.resolve(TypeIdentifierMap));
       return;
     }
-    for (Metadata *D : DCT->getElements()->operands()) {
+    for (Metadata *D : DCT->getElements()) {
       if (DIType T = dyn_cast<MDType>(D))
         processType(T);
       else if (DISubprogram SP = dyn_cast<MDSubprogram>(D))
         processSubprogram(SP);
     }
   } else if (DIDerivedType DDT = dyn_cast<MDDerivedTypeBase>(DT)) {
     processType(DDT.getTypeDerivedFrom().resolve(TypeIdentifierMap));
   }

llvm/trunk/test/DebugInfo/debuginfofinder-forward-declaration.ll

+; RUN: opt -analyze -module-debuginfo < %s | FileCheck %s
+
+
+; This module is generated from the following c-code:
+;
+; > union X;
+; >
+; > struct Y {
+; >     union X *x;
+; > };
+; >
+; > struct Y y;
+
+
+; CHECK: Type: Y from /tmp/minimal.c:3 DW_TAG_structure_type
+; CHECK: Type: x from /tmp/minimal.c:4 DW_TAG_member
+; CHECK: Type: DW_TAG_pointer_type
+; CHECK: Type: X from /tmp/minimal.c:1 DW_TAG_structure_type
+
+
+%struct.Y = type { %struct.X* }
+%struct.X = type opaque
+
+@y = common global %struct.Y zeroinitializer, align 8
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!10, !11}
+!llvm.ident = !{!12}
+
+!0 = !MDCompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 3.7.0 (http://llvm.org/git/clang.git 247b30a043eb8f39ea3708e7e995089da0a6b00f) (http://llvm.org/git/llvm.git 6ecc7365a89c771fd229bdd9ffcc178684ea1aa5)", isOptimized: false, runtimeVersion: 0, emissionKind: 1, enums: !2, retainedTypes: !2, subprograms: !2, globals: !3, imports: !2)
+!1 = !MDFile(filename: "minimal.c", directory: "/tmp")
+!2 = !{}
+!3 = !{!4}
+!4 = !MDGlobalVariable(name: "y", scope: !0, file: !1, line: 7, type: !5, isLocal: false, isDefinition: true, variable: %struct.Y* @y)
+!5 = !MDCompositeType(tag: DW_TAG_structure_type, name: "Y", file: !1, line: 3, size: 64, align: 64, elements: !6)
+!6 = !{!7}
+!7 = !MDDerivedType(tag: DW_TAG_member, name: "x", scope: !5, file: !1, line: 4, baseType: !8, size: 64, align: 64)
+!8 = !MDDerivedType(tag: DW_TAG_pointer_type, baseType: !9, size: 64, align: 64)
+!9 = !MDCompositeType(tag: DW_TAG_structure_type, name: "X", file: !1, line: 1, flags: DIFlagFwdDecl)
+!10 = !{i32 2, !"Dwarf Version", i32 4}
+!11 = !{i32 2, !"Debug Info Version", i32 3}
+!12 = !{!"clang version 3.7.0 (http://llvm.org/git/clang.git 247b30a043eb8f39ea3708e7e995089da0a6b00f) (http://llvm.org/git/llvm.git 6ecc7365a89c771fd229bdd9ffcc178684ea1aa5)"}