Use nested namespaces for IOS HLE

Pretty much all of the source files contain the following:

    namespace IOS
    {
    namespace HLE
    {
    namespace <name>
    {
    // actual code here
    }  // namespace <name>
    }  // namespace HLE
    }  // namespace IOS

which is really verbose boilerplate, because most of the files inside
of Core/IOS are for IOS HLE.

This commit replaces that with a more concise `namespace IOS::HLE`
or `namespace IOS::HLE::(name)`.

Source/Core/Core/IOS/DeviceStub.cpp

@@ -6,11 +6,7 @@
 
 #include "Common/Logging/Log.h"
 
-namespace IOS
-{
-namespace HLE
-{
-namespace Device
+namespace IOS::HLE::Device
 {
 IPCCommandResult Stub::Open(const OpenRequest& request)
 {
@@ -30,6 +26,4 @@ IPCCommandResult Stub::IOCtlV(const IOCtlVRequest& request)
   WARN_LOG(IOS, "%s faking IOCtlV()", m_name.c_str());
   return GetDefaultReply(IPC_SUCCESS);
 }
-}  // namespace Device
-}  // namespace HLE
-}  // namespace IOS
+}  // namespace IOS::HLE::Device