Rename semantic to sem

* Rename namespace semantic to sem
* Rename directory src/semantic/ to src/sem/

Bug: tint:724
Change-Id: I76383b821fbca7f1037a803c497b595a706dcb06
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/48120
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
diff --git a/src/resolver/storage_class_validation_test.cc b/src/resolver/storage_class_validation_test.cc
index 0fde1a1..5f9324e 100644
--- a/src/resolver/storage_class_validation_test.cc
+++ b/src/resolver/storage_class_validation_test.cc
@@ -16,7 +16,7 @@
 
 #include "gmock/gmock.h"
 #include "src/resolver/resolver_test_helper.h"
-#include "src/semantic/struct.h"
+#include "src/sem/struct.h"
 #include "src/type/access_control_type.h"
 
 namespace tint {