Changing compiler namespaces to the new single-line syntax. (#15827)

Some files already do this and now all do for consistency.
diff --git a/compiler/plugins/input/TOSA/tosa-iree/InputConversion/StripSignedness.cpp b/compiler/plugins/input/TOSA/tosa-iree/InputConversion/StripSignedness.cpp
index 787626d..c133a98 100644
--- a/compiler/plugins/input/TOSA/tosa-iree/InputConversion/StripSignedness.cpp
+++ b/compiler/plugins/input/TOSA/tosa-iree/InputConversion/StripSignedness.cpp
@@ -10,8 +10,7 @@
 #include "tosa-iree/InputConversion/PassDetail.h"
 #include "tosa-iree/InputConversion/Passes.h"
 
-namespace mlir {
-namespace iree_compiler {
+namespace mlir::iree_compiler {
 
 namespace {
 
@@ -131,5 +130,4 @@
   return std::make_unique<StripSignednessPass>();
 }
 
-} // namespace iree_compiler
-} // namespace mlir
+} // namespace mlir::iree_compiler