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/compound_assignment_statement_test.cc b/src/tint/ast/compound_assignment_statement_test.cc
index c84faf0..080f93e 100644
--- a/src/tint/ast/compound_assignment_statement_test.cc
+++ b/src/tint/ast/compound_assignment_statement_test.cc
@@ -17,8 +17,7 @@
 #include "gtest/gtest-spi.h"
 #include "src/tint/ast/test_helper.h"
 
-namespace tint {
-namespace ast {
+namespace tint::ast {
 namespace {
 
 using CompoundAssignmentStatementTest = TestHelper;
@@ -98,5 +97,4 @@
 }
 
 }  // namespace
-}  // namespace ast
-}  // namespace tint
+}  // namespace tint::ast