Sink DwarfUnit::Skeleton down into DwarfCompileUnit

Type units no longer have skeletons and it's misleading to be able to
query for a type unit's skeleton (it might incorrectly lead one to
conclude that if a unit doesn't have a skeleton it's not in a .dwo
file... ).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221055 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/AsmPrinter/DwarfCompileUnit.cpp

@@ -18,7 +18,8 @@ namespace llvm {
 DwarfCompileUnit::DwarfCompileUnit(unsigned UID, DICompileUnit Node,
                                    AsmPrinter *A, DwarfDebug *DW,
                                    DwarfFile *DWU)
-    : DwarfUnit(UID, dwarf::DW_TAG_compile_unit, Node, A, DW, DWU) {
+    : DwarfUnit(UID, dwarf::DW_TAG_compile_unit, Node, A, DW, DWU),
+      Skeleton(nullptr) {
   insertDIE(Node, &getUnitDie());
 }