Be sure to initialize DwarfCompileUnit::LabelBegin now that it may be skipped in initSection

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221079 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/AsmPrinter/DwarfCompileUnit.cpp

@@ -19,7 +19,7 @@ DwarfCompileUnit::DwarfCompileUnit(unsigned UID, DICompileUnit Node,
                                    AsmPrinter *A, DwarfDebug *DW,
                                    DwarfFile *DWU)
     : DwarfUnit(UID, dwarf::DW_TAG_compile_unit, Node, A, DW, DWU),
-      Skeleton(nullptr) {
+      Skeleton(nullptr), LabelBegin(nullptr) {
   insertDIE(Node, &getUnitDie());
 }