Fix bitcode upgrade for DIGlobalVariables with a var: field.

[aprantl]

Summary

This is a follow-up to https://reviews.llvm.org/D29349.
It turns out that NeedUpgradeToDIGlobalVariableExpression is always necessary when we encountered a version==0 record because it may always be referenced via a list of globals in a DICompileUnit. My tests weren't good enough to catch this though. To trigger this case, we need much older bitcode produced by LLVM around version 3.7.

[mehdi_amini]

Great: at least we increased the coverage :)

lib/Bitcode/Reader/MetadataLoader.cpp

if (Version == 1) {
                            getMDOrNull(Record[4]), Record[5],
                            getDITypeRefOrNull(Record[6]), Record[7], Record[8],
                            getMDOrNull(Record[10]), Record[11])),
           NextMetadataNo);
       NextMetadataNo++;
     } else if (Version == 0) {
       // Upgrade old metadata, which stored a global variable reference or a
       // ConstantInt here.
+      NeedUpgradeToDIGlobalVariableExpression = true;
       Metadata *Expr = getMDOrNull(Record[9]);
       uint32_t AlignInBits = 0;
       if (Record.size() > 11) {
         if (Record[11] > (uint64_t)std::numeric_limits<uint32_t>::max())
           return error("Alignment value is too large");
         AlignInBits = Record[11];
       }
       GlobalVariable *Attach = nullptr;
if (Version == 1) {
           (Context, getMDOrNull(Record[1]), getMDString(Record[2]),
            getMDString(Record[3]), getMDOrNull(Record[4]), Record[5],
            getDITypeRefOrNull(Record[6]), Record[7], Record[8],
            getMDOrNull(Record[10]), AlignInBits));
 
       DIGlobalVariableExpression *DGVE = nullptr;
       if (Attach || Expr)
         DGVE = DIGlobalVariableExpression::getDistinct(Context, DGV, Expr);
-      else
-        NeedUpgradeToDIGlobalVariableExpression = true;
       if (Attach)
         Attach->addDebugInfo(DGVE);
 
       auto *MDNode = Expr ? cast<Metadata>(DGVE) : cast<Metadata>(DGV);
       MetadataList.assignValue(MDNode, NextMetadataNo);
       NextMetadataNo++;
     } else
       return error("Invalid record");

test/Bitcode/DIGlobalVariableExpression2.ll

+; RUN: llvm-dis -o - %s.bc | FileCheck %s
+
+; CHECK: @g = common global i32 0, align 4, !dbg ![[G:[0-9]+]]
+; CHECK-DAG: ![[G]] = distinct !DIGlobalVariableExpression(var: ![[GVAR:[0-9]+]])
+; CHECK-DAG: distinct !DICompileUnit({{.*}}, globals: ![[GLOBS:[0-9]+]]
+; CHECK-DAG: ![[GLOBS]] = !{![[GEXPR:[0-9]+]]}
+; CHECK-DAG: ![[GEXPR]] = distinct !DIGlobalVariableExpression(var: ![[GVAR]])
+; CHECK-DAG: ![[GVAR]] = !DIGlobalVariable(name: "g",
+
+; Test the bitcode upgrade for DIGlobalVariable -> DIGlobalVariableExpression.
+
+; ModuleID = 'a.c'
+target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-apple-macosx10.12.0"
+
+@g = common global i32 0, align 4
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!6, !7, !8}
+!llvm.ident = !{!9}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 3.7.0 (clang-stage1-configure-RA_build 241111)", isOptimized: false, runtimeVersion: 0, emissionKind: 1, enums: !2, retainedTypes: !2, subprograms: !2, globals: !3, imports: !2)
+!1 = !DIFile(filename: "a.c", directory: "/tmp")
+!2 = !{}
+!3 = !{!4}
+!4 = !DIGlobalVariable(name: "g", scope: !0, file: !1, line: 1, type: !5, isLocal: false, isDefinition: true, variable: i32* @g)
+!5 = !DIBasicType(name: "int", size: 32, align: 32, encoding: DW_ATE_signed)
+!6 = !{i32 2, !"Dwarf Version", i32 2}
+!7 = !{i32 2, !"Debug Info Version", i32 3}
+!8 = !{i32 1, !"PIC Level", i32 2}
+!9 = !{!"clang version 3.7.0 (clang-stage1-configure-RA_build 241111)"}