Introduce LoopStmt
Convert While/For loops to inherit from LoopStmt
Convert BlockStmt break/continue label to protected variables and add virtual methods
Introduce functions isLoopStmt/toLoopStmt() and update virtual method BlockStmt::isLoop()
Merge pull request #979 from noakesmichael/loop-0

Introduce LoopStmt as a base class for While/For stmts

1) Introduce a new class LoopStmt to serve as a base class for While/For loops

2) Convert BlockStmt breakLabel/continueLabel to protected member variables of BlockStmt
and add virtual methods to access them.

3) Introduce the functions isLoopStmt() and toLoopStmt()

4) Convert the virtual method isLoop() to be isLoopStmt() and update the references.

Compiled: STD clang/darwin, STD gcc/linux64, LLVM gcc/linux64
Tested: paratest --verify
Review: none