Yes, .NET 4.5 has a brand spanking new version of the CLR, you can read about the improvements at;
http://blogs.microsoft.co.il/blogs/sasha/archive/2011/09/17/improvements-in-the-clr-core-in-net-framework-4-5.aspx
To clarify; this is a new version of the CLR that actually replaces the 4.0 one, so whether to call it an update or a new CLR is disputable.
To tell which CLR version you’re running under, use
System.Runtime.InteropServices.RuntimeEnvironment.GetSystemVersion()