How to Debug Mscorlib.dll in Visual Studio 2015

I could do reverse engineer of other custom dlls (my own created .net assemblies) successfully but I couldn't do it on mscorlib.dll with the same steps.

I have tried to generate pdb of mscorlib.dll using jetbrains dotpeek.

My visual studio debugging option has been set to the folder containing that pdb.

I tried to put the mscorlib.ini file to set the code Optimization to No, like this:

[.NET Framework Debugging Control]  
GenerateTrackingInfo=1  
AllowOptimize=0 

and put the file on the same location where the temporary dll is loaded (e.g.

C:\Windows\Microsoft.NET\assembly\GAC_64\mscorlib\v4.0_4.0.0.0__b77a5c561934e089).

When I attach my VS to a process, I could see that the Optimized value is still 'Yes' in Visual Studio's Modules.

Appreciate if anyone can guide me the step by step instructions on how to debug the mscorlib.dll when attaching VS to a process using mscorlib.dll.

Thanks.