Now that these files are mostly preprocessor directives, splitting makes sense.

(cherry picked from commit e04fdb7bf25ba5c02a8b86287e76d25dbea9e755)

rootcling now links against libRIO, libCore, libCling.
Any symbol from llvm/clang must now be resolved from within libCling.
libCling thus exports only ROOT_rootcling_Driver and CreateInterpreter().
This will allow to fix the symbol visibility inconsistency.
And it reduces link time.

Allows rootcling to find it whatever rtti etc setting are.

Split IO part of metacling/rootclingTCling.cxx into utils/rootclingIO.cxx; the rest depends is meta not cling.

Cling should never see this; add comment about libCore/libCling use; constness; default ctor = default.

This file is needed by libCling (dictgen) *and* libCore (TClass/TSchemaRule).
Luckily it is inline-only, so we simply #include it from both.

Basic, ROOT-independent interfaces (stringview, TClassEdit) go to core/foundation.
metautils becomes clingutils; hide TClingUtils (former TMetaUtils) from include/.
TCling* move to core/metacling/src/.
I/O related code for rootcling moves to core/metacling/.
Other dictionary-generation related code moves to core/dictgen.

- in group TMVA
- remove trailing spaces
- spell check
- math formula

- in group TMVA
- remove trailing spaces
- spell check

- in group TMVA
- remove trailing spaces
- spell check