Revert "[IR] Remove the DIExpression field from DIGlobalVariable."

This reverts commit 289920 (again).
I forgot to implement a Bitcode upgrade for the case where a DIGlobalVariable
has not DIExpression. Unfortunately it is not possible to safely upgrade
these variables without adding a flag to the bitcode record indicating which
version they are.
My plan of record is to roll the planned follow-up patch that adds a
unit: field to DIGlobalVariable into this patch before recomitting.
This way we only need one Bitcode upgrade for both changes (with a
version flag in the bitcode record to safely distinguish the record
formats).

Sorry for the churn!

llvm-svn: 289982
diff --git a/llvm/test/DebugInfo/X86/debug-info-static-member.ll b/llvm/test/DebugInfo/X86/debug-info-static-member.ll
index 55d6588..017501e 100644
--- a/llvm/test/DebugInfo/X86/debug-info-static-member.ll
+++ b/llvm/test/DebugInfo/X86/debug-info-static-member.ll
@@ -67,7 +67,7 @@
 !8 = !{!9}
 !9 = !DIBasicType(tag: DW_TAG_base_type, name: "int", size: 32, align: 32, encoding: DW_ATE_signed)
 !10 = !{!12, !27, !28}
-!12 = !DIGlobalVariableExpression(var: !DIGlobalVariable(name: "a", linkageName: "_ZN1C1aE", line: 14, isLocal: false, isDefinition: true, scope: null, file: !6, type: !9, declaration: !15))
+!12 = !DIGlobalVariable(name: "a", linkageName: "_ZN1C1aE", line: 14, isLocal: false, isDefinition: true, scope: null, file: !6, type: !9, declaration: !15)
 !13 = !DICompositeType(tag: DW_TAG_class_type, name: "C", line: 1, size: 32, align: 32, file: !33, elements: !14)
 !14 = !{!15, !16, !19, !20, !23, !24, !26}
 !15 = !DIDerivedType(tag: DW_TAG_member, name: "a", line: 3, flags: DIFlagPrivate | DIFlagStaticMember, file: !33, scope: !13, baseType: !9)
@@ -82,8 +82,8 @@
 !24 = !DIDerivedType(tag: DW_TAG_member, name: "const_c", line: 10, flags: DIFlagPublic | DIFlagStaticMember, file: !33, scope: !13, baseType: !25, extraData: i32 18)
 !25 = !DIDerivedType(tag: DW_TAG_const_type, baseType: !9)
 !26 = !DIDerivedType(tag: DW_TAG_member, name: "d", line: 11, size: 32, align: 32, flags: DIFlagPublic, file: !33, scope: !13, baseType: !9)
-!27 = !DIGlobalVariableExpression(var: !DIGlobalVariable(name: "b", linkageName: "_ZN1C1bE", line: 15, isLocal: false, isDefinition: true, scope: null, file: !6, type: !9, declaration: !19))
-!28 = !DIGlobalVariableExpression(var: !DIGlobalVariable(name: "c", linkageName: "_ZN1C1cE", line: 16, isLocal: false, isDefinition: true, scope: null, file: !6, type: !9, declaration: !23))
+!27 = !DIGlobalVariable(name: "b", linkageName: "_ZN1C1bE", line: 15, isLocal: false, isDefinition: true, scope: null, file: !6, type: !9, declaration: !19)
+!28 = !DIGlobalVariable(name: "c", linkageName: "_ZN1C1cE", line: 16, isLocal: false, isDefinition: true, scope: null, file: !6, type: !9, declaration: !23)
 !29 = !DILocalVariable(name: "instance_C", line: 20, scope: !5, file: !6, type: !13)
 !30 = !DILocation(line: 20, scope: !5)
 !31 = !DILocation(line: 21, scope: !5)