Removes IRLoader, unittests, and public interface

This unit test hasn't really served any purpose for a while now and
mostly just causes pain when reworking things in the IR.

Just remove the IRLoader, its unit tests, the github action steps and
the public FEXCore interface to it. Since it isn't used by anything
other than Thunks.

Also moves some IR definitions from the public API to the backend.

FEXCore/Source/Interface/IR/Passes/IRDumperPass.cpp

@@ -6,6 +6,7 @@ desc: Prints IR
 $end_info$
 */
 
+#include "Interface/IR/IR.h"
 #include "Interface/IR/PassManager.h"
 #include "Interface/IR/Passes/RegisterAllocationPass.h"
 #include "Interface/Core/OpcodeDispatcher.h"