[tree] Ensure branch names are C++ variable names

* [tree] valid branch names or filenames does not ensure valid C++ variable names

So convert wherever need in produced selector code

Fixes https://its.cern.ch/jira/browse/ROOT-8775

* simplify

thanks to silverweed

* [tree] reuse map-cpp-name function from clingutils

as suggested by dpiparo

* [tree] clarify variable names

as suggested by pcanal

* [tree] clarify variable names

as suggested by pcanal

* [core] clarify warning

tree/treeplayer/inc/TTreeReaderGenerator.h

@@ -33,6 +33,8 @@ class TLeaf;
 namespace ROOT {
 namespace Internal {
 
+   TString GetCppName(TString name);
+
    /// 0 for the general case, 1 when this a split clases inside a TClonesArray,
    /// 2 when this is a split classes inside an STL container.
    enum ELocation { kOut=0, kClones, kSTL };