Visit previously unreachable nodes in the debug info metadata verifier.

This allows for diagnosing malformed LLVM IR debug info metadata such
as the one in the testcase.

<rdar://problem/59756060>

Differential Revision: https://reviews.llvm.org/D75212

llvm/lib/IR/Verifier.cpp

@@ -2398,6 +2398,7 @@ void Verifier::visitFunction(const Function &F) {
     AssertDI(SP->describes(&F),
              "!dbg attachment points at wrong subprogram for function", N, &F,
              &I, DL, Scope, SP);
+    visitMDNode(*SP);
   };
   for (auto &BB : F)
     for (auto &I : BB) {

llvm/test/Verifier/llvm.loop.cu.ll

@@ -0,0 +1,26 @@
+; RUN: llvm-as -disable-output < %s -o /dev/null 2>&1 | FileCheck %s
+
+define void @f()  #0 !dbg !6 {
+  br label %1, !dbg !9, !llvm.loop !10
+  ret void
+}
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!2, !3, !4}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1)
+!1 = !DIFile(filename: "f.c", directory: "./")
+!2 = !{i32 2, !"Dwarf Version", i32 4}
+!3 = !{i32 2, !"Debug Info Version", i32 3}
+!4 = !{i32 7, !"PIC Level", i32 2}
+!6 = distinct !DISubprogram(name: "f", scope: !1, file: !1, line: 7, type: !7, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition, unit: !0)
+!7 = !DISubroutineType(types: !8)
+!8 = !{}
+!9 = !DILocation(line: 18, column: 2, scope: !6)
+!10 = distinct !{!10, !11}
+!11 = !DILocation(line: 18, column: 2, scope: !12)
+!12 = distinct !DISubprogram(name: "f", scope: !1, file: !1, line: 7, type: !7, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition, unit: !13)
+
+; CHECK: warning: ignoring invalid debug info
+; This CU isn't listed in llvm.dbg.cu
+!13 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1)