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>

src/sem/sem_expression.cc

@@ -0,0 +1,30 @@
+// Copyright 2021 The Tint Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "src/sem/expression.h"
+
+TINT_INSTANTIATE_TYPEINFO(tint::sem::Expression);
+
+namespace tint {
+namespace sem {
+
+Expression::Expression(ast::Expression* declaration,
+                       type::Type* type,
+                       Statement* statement)
+    : declaration_(declaration),
+      type_(type->UnwrapIfNeeded()),
+      statement_(statement) {}
+
+}  // namespace sem
+}  // namespace tint