Add move assignment operators to tint::ast::Module and tint::Context

This enables usage with DAWN_TRY_ASSIGN in Dawn in simplify error
handling.

Bug: tint:306
Change-Id: I91a96b6670341943c26d9972058ecc3ad8162db0
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/34640
Commit-Queue: dan sinclair <dsinclair@chromium.org>
Auto-Submit: Austin Eng <enga@chromium.org>
Reviewed-by: dan sinclair <dsinclair@chromium.org>

src/ast/module.cc

@@ -25,6 +25,7 @@ namespace ast {
 Module::Module() = default;
 
 Module::Module(Module&&) = default;
+Module& Module::operator=(Module&& rhs) = default;
 
 Module::~Module() = default;
 

src/ast/module.h

@@ -40,6 +40,8 @@ class Module {
   Module();
   /// Move constructor
   Module(Module&&);
+  // Move assignment
+  Module& operator=(Module&& rhs);
   ~Module();
 
   /// @return a deep copy of this module

src/ast/type_manager.cc

@@ -21,6 +21,7 @@ namespace ast {
 
 TypeManager::TypeManager() = default;
 TypeManager::TypeManager(TypeManager&&) = default;
+TypeManager& TypeManager::operator=(TypeManager&& rhs) = default;
 TypeManager::~TypeManager() = default;
 
 void TypeManager::Reset() {

src/ast/type_manager.h

@@ -31,6 +31,8 @@ class TypeManager {
   TypeManager();
   /// Move constructor
   TypeManager(TypeManager&&);
+  // Move assignment
+  TypeManager& operator=(TypeManager&& rhs);
   ~TypeManager();
 
   /// Clears all registered types.