[Cloning] Move distinct GlobalVariable debug info metadata in CloneModule

Duplicating the distinct Subprogram and CU metadata nodes seems like the incorrect thing to do in CloneModule for GlobalVariable debug info. As it results in the scope of the GlobalVariable DI no longer being consistent with the rest of the module, and the new CU is absent from llvm.dbg.cu.

Fixed by adding RF_MoveDistinctMDs to MapMetadata flags for GlobalVariables.

Current unit test IR after clone:
```
@gv = global i32 1, comdat($comdat), !dbg !0, !type !5

define private void @f() comdat($comdat) personality void ()* @persfn !dbg !14 {

!llvm.dbg.cu = !{!10}

!0 = !DIGlobalVariableExpression(var: !1)
!1 = distinct !DIGlobalVariable(name: "gv", linkageName: "gv", scope: !2, file: !3, line: 1, type: !9, isLocal: false, isDefinition: true)
!2 = distinct !DISubprogram(name: "f", linkageName: "f", scope: null, file: !3, line: 4, type: !4, isLocal: true, isDefinition: true, scopeLine: 3, isOptimized: false, unit: !6, variables: !5)
!3 = !DIFile(filename: "filename.c", directory: "/file/dir/")
!4 = !DISubroutineType(types: !5)
!5 = !{}
!6 = distinct !DICompileUnit(language: DW_LANG_C99, file: !7, producer: "CloneModule", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !5, globals: !8)
!7 = !DIFile(filename: "filename.c", directory: "/file/dir")
!8 = !{!0}
!9 = !DIBasicType(tag: DW_TAG_unspecified_type, name: "decltype(nullptr)")
!10 = distinct !DICompileUnit(language: DW_LANG_C99, file: !7, producer: "CloneModule", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !5, globals: !11)
!11 = !{!12}
!12 = !DIGlobalVariableExpression(var: !13)
!13 = distinct !DIGlobalVariable(name: "gv", linkageName: "gv", scope: !14, file: !3, line: 1, type: !9, isLocal: false, isDefinition: true)
!14 = distinct !DISubprogram(name: "f", linkageName: "f", scope: null, file: !3, line: 4, type: !4, isLocal: true, isDefinition: true, scopeLine: 3, isOptimized: false, unit: !10, variables: !5)
```

Patched IR after clone:
```
@gv = global i32 1, comdat($comdat), !dbg !0, !type !5

define private void @f() comdat($comdat) personality void ()* @persfn !dbg !2 {

!llvm.dbg.cu = !{!6}

!0 = !DIGlobalVariableExpression(var: !1)
!1 = distinct !DIGlobalVariable(name: "gv", linkageName: "gv", scope: !2, file: !3, line: 1, type: !9, isLocal: false, isDefinition: true)
!2 = distinct !DISubprogram(name: "f", linkageName: "f", scope: null, file: !3, line: 4, type: !4, isLocal: true, isDefinition: true, scopeLine: 3, isOptimized: false, unit: !6, variables: !5)
!3 = !DIFile(filename: "filename.c", directory: "/file/dir/")
!4 = !DISubroutineType(types: !5)
!5 = !{}
!6 = distinct !DICompileUnit(language: DW_LANG_C99, file: !7, producer: "CloneModule", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !5, globals: !8)
!7 = !DIFile(filename: "filename.c", directory: "/file/dir")
!8 = !{!0}
!9 = !DIBasicType(tag: DW_TAG_unspecified_type, name: "decltype(nullptr)")
```

Reviewers: aprantl, probinson, dblaikie, echristo, loladiro
Reviewed By: aprantl
Subscribers: llvm-commits
Differential Revision: https://reviews.llvm.org/D36082

llvm-svn: 309928