AsmDiff: unmanaged keyword getting confused with struct

[carlossanlop]

Two examples:

dotnet/core#9176 (comment)

public static ComVariant CreateRaw<T>(VarEnum vt, T rawValue) where T : struct;

Should've been:

public static ComVariant CreateRaw<T>(VarEnum vt, T rawValue) where T : unmanaged;

dotnet/core#9176 (comment)

public ref T GetRawDataRef<T>() where T : struct;

public ref T GetRawDataRef<T>() where T : unmanaged;