avoid having to manuall modify sources when adding a new clang plugin

Now each one registers in its .cxx file.

Change-Id: I811c0d4400c2bdccc1c287269378d7e8ad8743ce

compilerplugins/clang/removeforwardstringdecl.cxx

@@ -73,4 +73,6 @@ bool RemoveForwardStringDecl::tryRemoveStringForwardDecl( const Decl* decl )
     return false;
     }
 
+static Plugin::Registration< RemoveForwardStringDecl > X( "removeforwardstringdecl" );
+
 } // namespace