Refactor implementation of Statement class heirarchy to use statement block.
Use LLVM double-link with parent list to store statements within a block.

PiperOrigin-RevId: 204515541
diff --git a/lib/IR/Function.cpp b/lib/IR/Function.cpp
index 92c3981..453797f 100644
--- a/lib/IR/Function.cpp
+++ b/lib/IR/Function.cpp
@@ -48,5 +48,5 @@
 //===----------------------------------------------------------------------===//
 
 MLFunction::MLFunction(StringRef name, FunctionType *type)
-  : Function(name, type, Kind::MLFunc) {
+  : Function(name, type, Kind::MLFunc), StmtBlock() {
 }