Condense namespaces in tint/sem.

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

Change-Id: Iff802ca78d4cb7fd21134a640dfe5d04e85148b4
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/86030
Commit-Queue: Dan Sinclair <dsinclair@chromium.org>
Auto-Submit: Dan Sinclair <dsinclair@chromium.org>
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
diff --git a/src/tint/sem/node.h b/src/tint/sem/node.h
index ecf630d..d03b042 100644
--- a/src/tint/sem/node.h
+++ b/src/tint/sem/node.h
@@ -17,8 +17,7 @@
 
 #include "src/tint/castable.h"
 
-namespace tint {
-namespace sem {
+namespace tint::sem {
 
 /// Node is the base class for all semantic nodes
 class Node : public Castable<Node> {
@@ -33,7 +32,6 @@
   ~Node() override;
 };
 
-}  // namespace sem
-}  // namespace tint
+}  // namespace tint::sem
 
 #endif  // SRC_TINT_SEM_NODE_H_