Move IsStdClass to cling::utils::Analyze
Always go through a TClass to search for base class.

To solve ROOT-6020, use TClass::GetClass to resolve the string
being passed so that we completely and properly resolve the
classname (and reduce the cost of the search).
Introduce Transform::Config::DropDefaultArg

Currently only implemented for the STL collection.

This could eventually be used to allow customization
of which default argument to strip and which class template.

For now, it is only used to prevent the addition of template
default template argument to the STL collection ... which
we would any strip.

This fixes the secondary issue in ROOT-6020 (one class template
instance having more than one corresponding TClass due to the
inconsistency in the normalization (not adding vs not stripping)
Speed-up THtml.

It has been slowed down by the change in TClass::GetBaseClass which
now calls TClass::GetClass (the time is spend in the clang lookup/search).

Remove the optimization in TClass::InheritsFrom(const char *classname)
as it no longer make any difference (TClass::GetClass called in both case)