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/expression.cc b/src/tint/ast/expression.cc
index a432326..a7f23aa 100644
--- a/src/tint/ast/expression.cc
+++ b/src/tint/ast/expression.cc
@@ -19,8 +19,7 @@
 
 TINT_INSTANTIATE_TYPEINFO(tint::ast::Expression);
 
-namespace tint {
-namespace ast {
+namespace tint::ast {
 
 Expression::Expression(ProgramID pid, const Source& src) : Base(pid, src) {}
 
@@ -28,5 +27,4 @@
 
 Expression::~Expression() = default;
 
-}  // namespace ast
-}  // namespace tint
+}  // namespace tint::ast