[mlir] Add initial graphdef->mlir generation

This CL adds:
  * One graphdef extracted from the TF test suite. More will come.
  * Scaffolding for the "graphdef2mlir" tool.
  * Importing of simple graphs. Type inference is not yet working, and attributes do not work either.
  * A fix for CFGFunction::~CFGFunction to not die if the function was destroyed without a terminator (for example if we exit early due to an error).

PiperOrigin-RevId: 206965992

mlir/lib/IR/Function.cpp

@@ -109,7 +109,8 @@ CFGFunction::~CFGFunction() {
   for (auto &bb : *this) {
     for (auto &inst : bb)
       inst.dropAllReferences();
-    bb.getTerminator()->dropAllReferences();
+    if (bb.getTerminator())
+      bb.getTerminator()->dropAllReferences();
   }
 }