Add more comments on the implementation of IsOnHeap (see ROOT-6805).

In particular note the known (intentional/impreventable) uninitialized memory read and its
very rare (but very visible) potential consequence.
Extend comment (describe argument meaning)
Remove redundant/obsolete calls to CheckClassInfo.

In addition to be redundant, those calls were provoking unwanted autoloading/autoparsing.
This fixes a crash due to autoloading being executed at tear down time seen on the cms builds.
Make the autoloading param of CheckClassInof non-default/explicit
Fix windows builds