[Evaluator] GraphViz printing for all of the dependencies.

Introduce another form of debugging dump for the evaluator, rendering the
complete dependency graph using GraphViz, including all dependencies and
values cached within the evaluator.

lib/Basic/AnyValue.cpp

@@ -18,3 +18,12 @@
 using namespace swift;
 
 AnyValue::HolderBase::~HolderBase() { }
+
+std::string AnyValue::getAsString() const {
+  std::string result;
+  {
+    llvm::raw_string_ostream out(result);
+    simple_display(out, *this);
+  }
+  return result;
+}