Make getCompileUnit non-const and return the current DIE if it

happens to be a compile unit. Noticed on inspection and tested
via calling on a newly created compile unit. No functional change.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181835 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/AsmPrinter/DIE.cpp

@@ -114,8 +114,8 @@ DIE::~DIE() {
 
 /// Climb up the parent chain to get the compile unit DIE to which this DIE
 /// belongs.
-DIE *DIE::getCompileUnit() const {
-  DIE *p = getParent();
+DIE *DIE::getCompileUnit() {
+  DIE *p = this;
   while (p) {
     if (p->getTag() == dwarf::DW_TAG_compile_unit)
       return p;