Be more explicit about AST types (#921)

This was born out of wanting FunctionDeclaration to explicitly have a Block for a body, and became a bit more of specifying types around. Note this forces exec_program to generate a Block for print()'s body, which is probably more correct as now we can expect a standard FunctionDeclaration AST structure, even for the built-ins.

There is a syntactic change here: a continuation's body is now a Block, not just a Statement. I've added an example disallowed test case. I think this is more reasonable syntax.

Other than that, note that optional_else now generates a valid Block. This has me thinking about whether we can eliminate Sequence, but that seemed well out of scope for this.

executable_semantics/ast/expression.h

@@ -79,19 +79,6 @@ class Expression {
   std::optional<Nonnull<const Value*>> static_type_;
 };
 
-// Converts paren_contents to an Expression, interpreting the parentheses as
-// grouping if their contents permit that interpretation, or as forming a
-// tuple otherwise.
-auto ExpressionFromParenContents(
-    Nonnull<Arena*> arena, SourceLocation source_loc,
-    const ParenContents<Expression>& paren_contents) -> Nonnull<Expression*>;
-
-// Converts paren_contents to an Expression, interpreting the parentheses as
-// forming a tuple.
-auto TupleExpressionFromParenContents(
-    Nonnull<Arena*> arena, SourceLocation source_loc,
-    const ParenContents<Expression>& paren_contents) -> Nonnull<Expression*>;
-
 // A FieldInitializer represents the initialization of a single struct field.
 class FieldInitializer {
  public:
@@ -450,6 +437,19 @@ class IntrinsicExpression : public Expression {
   Intrinsic intrinsic_;
 };
 
+// Converts paren_contents to an Expression, interpreting the parentheses as
+// grouping if their contents permit that interpretation, or as forming a
+// tuple otherwise.
+auto ExpressionFromParenContents(
+    Nonnull<Arena*> arena, SourceLocation source_loc,
+    const ParenContents<Expression>& paren_contents) -> Nonnull<Expression*>;
+
+// Converts paren_contents to an Expression, interpreting the parentheses as
+// forming a tuple.
+auto TupleExpressionFromParenContents(
+    Nonnull<Arena*> arena, SourceLocation source_loc,
+    const ParenContents<Expression>& paren_contents) -> Nonnull<TupleLiteral*>;
+
 }  // namespace Carbon
 
 #endif  // EXECUTABLE_SEMANTICS_AST_EXPRESSION_H_