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>

src/tint/sem/parameter_usage.cc.tmpl

diff --git a/src/tint/sem/parameter_usage.cc.tmpl b/src/tint/sem/parameter_usage.cc.tmpl
index 2a9a060..1339171 100644
--- a/src/tint/sem/parameter_usage.cc.tmpl
+++ b/src/tint/sem/parameter_usage.cc.tmpl
@@ -10,8 +10,7 @@
 
 #include "src/tint/sem/parameter_usage.h"
 
-namespace tint {
-namespace sem {
+namespace tint::sem {
 
 const char* str(ParameterUsage usage) {
   switch (usage) {
@@ -25,5 +24,4 @@
   return "<unknown>";
 }
 
-}  // namespace sem
-}  // namespace tint
+}  // namespace tint::sem