Fix crash at translation of Entity of DebugImportedEntity (#951)

This is workaround to fix crash, which caused by using `dyn_cast` on a `nullptr`,
as the result of translation of `Entity` operand of `DebugImportedEntity`, because
this operand is `DebugInfoNone`.

Author: KornevNikita

lib/SPIRV/SPIRVToLLVMDbgTran.cpp

@@ -769,6 +769,9 @@ DINode *SPIRVToLLVMDbgTran::transImportedEntry(const SPIRVExtInst *DebugInst) {
   DIFile *File = getFile(Ops[SourceIdx]);
   auto *Entity = transDebugInst<DINode>(BM->get<SPIRVExtInst>(Ops[EntityIdx]));
   if (Ops[TagIdx] == SPIRVDebug::ImportedModule) {
+    if (!Entity)
+      return Builder.createImportedModule(
+          Scope, static_cast<DIImportedEntity *>(nullptr), File, Line);
     if (DIImportedEntity *IE = dyn_cast<DIImportedEntity>(Entity))
       return Builder.createImportedModule(Scope, IE, File, Line);
     if (DINamespace *NS = dyn_cast<DINamespace>(Entity))

test/DebugInfo/DebugInfoNoneEntity.ll

@@ -0,0 +1,26 @@
+; RUN: llvm-as %s -o %t.bc
+; Translation shouldn't crash:
+; RUN: llvm-spirv %t.bc -spirv-text
+; RUN: llvm-spirv %t.bc -o %t.spv
+; RUN: llvm-spirv -r %t.spv -o %t.rev.bc
+
+source_filename = "llvm-link"
+target datalayout = "e-i64:64-v16:16-v24:32-v32:32-v48:64-v96:128-v192:256-v256:256-v512:512-v1024:1024-n8:16:32:64"
+target triple = "spir64"
+
+!llvm.module.flags = !{!1, !2, !3, !4}
+!llvm.dbg.cu = !{!5}
+
+!0 = !{}
+!1 = !{i32 1, !"wchar_size", i32 4}
+!2 = !{i32 7, !"PIC Level", i32 2}
+!3 = !{i32 2, !"Debug Info Version", i32 3}
+!4 = !{i32 2, !"Dwarf Version", i32 4}
+!5 = distinct !DICompileUnit(language: DW_LANG_Fortran95, file: !6, isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !0, globals: !0, imports: !7, splitDebugInlining: false, nameTableKind: None)
+!6 = !DIFile(filename: "declare_target_subroutine.F90", directory: "/test")
+!7 = !{!8}
+!8 = !DIImportedEntity(tag: DW_TAG_imported_module, scope: !9, entity: !12, file: !6, line: 24)
+!9 = distinct !DISubprogram(name: "declare_target_subroutine", linkageName: "MAIN__", scope: !6, file: !6, line: 23, type: !10, scopeLine: 23, spFlags: DISPFlagDefinition | DISPFlagMainSubprogram, unit: !5, retainedNodes: !0)
+!10 = !DISubroutineType(types: !11)
+!11 = !{null}
+!12 = !DIModule(scope: !9, name: "iso_fortran_env", isDecl: true)