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/ast/discard_statement.cc b/src/ast/discard_statement.cc
index 9a8b2fe..aa3ff08 100644
--- a/src/ast/discard_statement.cc
+++ b/src/ast/discard_statement.cc
@@ -34,7 +34,7 @@
   return ctx->dst->create<DiscardStatement>(src);
 }
 
-void DiscardStatement::to_str(const semantic::Info&,
+void DiscardStatement::to_str(const sem::Info&,
                               std::ostream& out,
                               size_t indent) const {
   make_indent(out, indent);