StmtIterator: Put stmt and decl group pointer into a union.

Reduce the size of StmtIterator without changing behavior.

llvm-svn: 241356
diff --git a/clang/lib/AST/StmtIterator.cpp b/clang/lib/AST/StmtIterator.cpp
index 1ccba04..732756f 100644
--- a/clang/lib/AST/StmtIterator.cpp
+++ b/clang/lib/AST/StmtIterator.cpp
@@ -93,12 +93,12 @@
 }
 
 StmtIteratorBase::StmtIteratorBase(Decl** dgi, Decl** dge)
-  : stmt(nullptr), DGI(dgi), RawVAPtr(DeclGroupMode), DGE(dge) {
+  : DGI(dgi), RawVAPtr(DeclGroupMode), DGE(dge) {
   NextDecl(false);
 }
 
 StmtIteratorBase::StmtIteratorBase(const VariableArrayType* t)
-  : stmt(nullptr), DGI(nullptr), RawVAPtr(SizeOfTypeVAMode) {
+  : DGI(nullptr), RawVAPtr(SizeOfTypeVAMode) {
   RawVAPtr |= reinterpret_cast<uintptr_t>(t);
 }