adapted code gen for Sierra For Stmt to be compatible with break/continue.
the break and continue jump dests has to be inserted as soon as we have access
to `CodeGenFunction's BreakContinueStack...; break and continue have not yet
been implemented
added cleanup exit block to sierra for
adapted Sierra while code gen for break / continue
declared Sierra's code gen functions for loops to be friend of the class
`CodeGenFunction', such that they can access the BreakContinueStack
extended Sierra code gen to add the correct break/continue targets to the
BreakContinueStack (codegen for while now creates an extra basic block)
added code gen for EmitScalarExpr of binary logical And
added code gen for EmitScalarExpr of binary logical Or