Factor out AST node for function return types. (#912)

This enables us to stop treating the return type as a Pattern (which is really isn't), treat return types more consistently with other static types in the typechecker, and drop ReturnTypeContext.

Additional changes:
- Merge TypeCheckFunDef with TypeOfFunDef.
- Handle implicit conversions in `return` statements.
- Require function type literals to have an explicit `->`.
- Move consistency check for omitted returns from TypeChecker to ResolveControlFlow.

executable_semantics/ast/declaration.cpp

@@ -45,6 +45,19 @@ void Declaration::Print(llvm::raw_ostream& out) const {
   }
 }
 
+void ReturnTerm::Print(llvm::raw_ostream& out) const {
+  switch (kind_) {
+    case ReturnKind::Omitted:
+      return;
+    case ReturnKind::Auto:
+      out << "-> auto";
+      return;
+    case ReturnKind::Expression:
+      out << "-> " << **type_expression_;
+      return;
+  }
+}
+
 void FunctionDeclaration::PrintDepth(int depth, llvm::raw_ostream& out) const {
   out << "fn " << name_ << " ";
   if (!deduced_parameters_.empty()) {
@@ -60,10 +73,7 @@ void FunctionDeclaration::PrintDepth(int depth, llvm::raw_ostream& out) const {
     }
     out << "]";
   }
-  out << *param_pattern_;
-  if (!is_omitted_return_type_) {
-    out << " -> " << *return_type_;
-  }
+  out << *param_pattern_ << return_term_;
   if (body_) {
     out << " {\n";
     (*body_)->PrintDepth(depth, out);