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/struct_member_align_attribute.cc b/src/tint/ast/struct_member_align_attribute.cc
index 7790800..8736dd10 100644
--- a/src/tint/ast/struct_member_align_attribute.cc
+++ b/src/tint/ast/struct_member_align_attribute.cc
@@ -21,8 +21,7 @@
 
 TINT_INSTANTIATE_TYPEINFO(tint::ast::StructMemberAlignAttribute);
 
-namespace tint {
-namespace ast {
+namespace tint::ast {
 
 StructMemberAlignAttribute::StructMemberAlignAttribute(ProgramID pid,
                                                        const Source& src,
@@ -42,5 +41,4 @@
   return ctx->dst->create<StructMemberAlignAttribute>(src, align);
 }
 
-}  // namespace ast
-}  // namespace tint
+}  // namespace tint::ast