Removed "ObserverTy" from core DataflowValues types. The observer

Removed "ObserverTy" from core DataflowValues types.  The observer
mechanism can be implemented simply by affixing the Observer to an
analysis meta data, so it doesn't need to be a required type.  This
also permits analyses not to implement an Observer if it doesn't make
sense.

Changed "DataflowValues::MetaDataTy" to
"DataflowValues::AnalysisDataTy" to reflect that the type
enscapsulated the data associated with analyzing a given CFG.

Changed CFGStmtVisitor::BlockStmt_VisitImplicitControlFlowStmt(Stmt*)
to ...VisitImplicitControlFlowExpr(Expr*).  The type narrowing is more
precise and more useful to clients.

Added CFGStmtVisitor::BlockStmt_VisitExpr to reflect the visitation of
expressions at the block statement level.  This captures all implicit
control-flow statements as well as other expressions that are hoisted
to the block level (such as conditions for terminators and function
calls).  This is especially useful for dataflow analysis.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@42034 91177308-0d34-0410-b5e6-96231b3b80d8