We need parseForModule(): we must not do codeGen on things that are in the library.

This flag will be used by the DeclCollector in a subsequent commit.
Improve performance of HandleNewDecl by testing easy and common things first.


git-svn-id: http://root.cern.ch/svn/root/trunk@49150 27541ba8-7e3a-0410-8455-c3a389f83636