Condense namespaces in tint/ast.

This PR condenses the namespaces in the tint/ast folder.

Change-Id: I77a84fb03f921b4db7135572005a08563f2fb60b
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/86038
Reviewed-by: Ben Clayton <bclayton@google.com>
Auto-Submit: Dan Sinclair <dsinclair@chromium.org>
Commit-Queue: Dan Sinclair <dsinclair@chromium.org>
diff --git a/src/tint/ast/member_accessor_expression.h b/src/tint/ast/member_accessor_expression.h
index 8a82ba1..800d018 100644
--- a/src/tint/ast/member_accessor_expression.h
+++ b/src/tint/ast/member_accessor_expression.h
@@ -17,8 +17,7 @@
 
 #include "src/tint/ast/identifier_expression.h"
 
-namespace tint {
-namespace ast {
+namespace tint::ast {
 
 /// A member accessor expression
 class MemberAccessorExpression final
@@ -50,7 +49,6 @@
   const IdentifierExpression* const member;
 };
 
-}  // namespace ast
-}  // namespace tint
+}  // namespace tint::ast
 
 #endif  // SRC_TINT_AST_MEMBER_ACCESSOR_EXPRESSION_H_