Created new path-sensitive bug-reporting scheme based on the classes

"BugReporter" and "BugDescription".  BugDescription is used to describe
a bug and provide pieces of the PathDiagnostic, and BugReporter creates
the actual PathDiagnostic by crawling through the ExplodedGraph.

Migrated checks done by GRSimpleVals to be reported using the new BugReporter
mechanism.

llvm-svn: 49127