Added static method "CFG::hasImplicitControlFlow".

This method is used to determine if an expression contains implicit
control-flow, and thus appears in a distinct statement slot in the CFG.

For example:

  (1) x = ... ? ... ? ...

  logically becomes:

  (1) ... ? ... : ...  (a unique statement slot for the ternary ?)
  (2) x = [E1]         (where E1 is actually the ConditionalOperator*)

A client of the CFG, when walking the statement at (2), will encounter
E1.  In this case, hasImplicitControlFlow(E1) == true, and the client
will know that the expression E1 is explicitly placed into its own statement
slot to capture the implicit control-flow it has.

llvm-svn: 41868