[ast] Add ast::Module::AddFunction()

Remove the non-const ast::Module::Functions() getter.

Change-Id: I365eece04837ee6bd51147d226c73e04ce5d9bd4
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/41300
Reviewed-by: Ben Clayton <bclayton@google.com>
Reviewed-by: dan sinclair <dsinclair@chromium.org>
Commit-Queue: James Price <jrprice@google.com>

src/ast/module.h

@@ -71,11 +71,12 @@ class Module : public Castable<Module, Node> {
     return constructed_types_;
   }
 
-  /// @returns the functions declared in the translation unit
-  const FunctionList& Functions() const { return functions_; }
+  /// Add a function to the Builder
+  /// @param func the function to add
+  void AddFunction(ast::Function* func) { functions_.push_back(func); }
 
   /// @returns the functions declared in the translation unit
-  FunctionList& Functions() { return functions_; }
+  const FunctionList& Functions() const { return functions_; }
 
   /// @returns true if all required fields in the AST are present.
   bool IsValid() const override;

src/ast/module_test.cc

@@ -117,7 +117,7 @@ TEST_F(ModuleTest, IsValid_Function) {
 }
 
 TEST_F(ModuleTest, IsValid_Null_Function) {
-  AST().Functions().Add(nullptr);
+  AST().AddFunction(nullptr);
   Program program(std::move(*this));
   EXPECT_FALSE(program.AST().IsValid());
 }