Support DWARF5 static member as DW_TAG_variable (#2229)

This resolves KhronosGroup/SPIRV-LLVM-Translator#2224

Original commit:
KhronosGroup/SPIRV-LLVM-Translator@0888551

Author: vmaksimo

llvm-spirv/test/DebugInfo/Generic/dwarf-public-names.ll

@@ -109,7 +109,7 @@ attributes #1 = { nounwind readnone }
 !2 = !DICompositeType(tag: DW_TAG_structure_type, name: "C", file: !3, line: 1, size: 8, align: 8, elements: !4)
 !3 = !DIFile(filename: "dwarf-public-names.cpp", directory: "/usr2/kparzysz/s.hex/t")
 !4 = !{!5, !7, !12}
-!5 = !DIDerivedType(tag: DW_TAG_member, name: "static_member_variable", scope: !2, file: !3, line: 4, baseType: !6, flags: DIFlagStaticMember)
+!5 = !DIDerivedType(tag: DW_TAG_variable, name: "static_member_variable", scope: !2, file: !3, line: 4, baseType: !6, flags: DIFlagStaticMember)
 !6 = !DIBasicType(name: "int", size: 32, align: 32, encoding: DW_ATE_signed)
 !7 = !DISubprogram(name: "member_function", linkageName: "_ZN1C15member_functionEv", scope: !2, file: !3, line: 2, type: !8, isLocal: false, isDefinition: false, scopeLine: 2, virtualIndex: 6, flags: DIFlagPrototyped, isOptimized: false, retainedNodes: !11)
 !8 = !DISubroutineType(types: !9)

llvm-spirv/test/DebugInfo/NonSemantic/static_member_array.ll

@@ -66,10 +66,10 @@ target triple = "spir64-unknown-unknown"
 !9 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
 !10 = !{!11}
 !11 = !DISubrange(count: 4)
-!12 = !DIDerivedType(tag: DW_TAG_member, name: "smem", scope: !13, file: !3, line: 4, baseType: !16, flags: DIFlagStaticMember)
+!12 = !DIDerivedType(tag: DW_TAG_variable, name: "smem", scope: !13, file: !3, line: 4, baseType: !16, flags: DIFlagStaticMember)
 !13 = distinct !DICompositeType(tag: DW_TAG_structure_type, name: "A", file: !3, line: 1, size: 8, elements: !14, identifier: "_ZTS1A")
 !14 = !{!15, !12}
-!15 = !DIDerivedType(tag: DW_TAG_member, name: "fully_specified", scope: !13, file: !3, line: 3, baseType: !9, flags: DIFlagStaticMember)
+!15 = !DIDerivedType(tag: DW_TAG_variable, name: "fully_specified", scope: !13, file: !3, line: 3, baseType: !9, flags: DIFlagStaticMember)
 !16 = !DICompositeType(tag: DW_TAG_array_type, baseType: !9, elements: !17)
 !17 = !{!18}
 !18 = !DISubrange(count: -1)