Simplify the RAII:

when constructing it takes a snapshot of whatever is relevant.
when destructing it restores whatever is relevant.

Don't mess with the diag Begin/EndSourceFile(); that's only relevant on a frontend level (aka are we parsing command line flgs or already source files?)


git-svn-id: http://root.cern.ch/svn/root/trunk@47474 27541ba8-7e3a-0410-8455-c3a389f83636