class_loader_imp: do not call virtual function in destructor

[cwecht]

This fixes clang-tidys "clang-analyzer-optin.cplusplus.VirtualCall"-warning.

Also this makes more clear, which function is actually called.

[nuclearsandwich]

Humbly begging your forgiveness @cwecht for the tardy review. I discussed this change with @clalancette and here is what we observed:

• The current destructor could result in a crash when the virtual function is called since the implementation is in the subclass whose destructor has already been run.
• With this patch, any subclass which overloads getBaseClassType will not have its implementation used here.

With those observations in mind, averting a crash is highly desirable but the tradeoff is that anyone out there overriding getBaseClassType won't have their implementation used by the debug message.

We discussed making getBaseClassType private and non-virtual but that only works if no one is actually using overrides.