Convert getFirstExpr(Expr* expr) to virtual Expr* getFirstExpr() so that new Loop ASTs can be handled more cleanly
Merge pull request #918 from noakesmichael/firstExpr

Convert getFirstExpr(Expr* expr) to virtual Expr* getFirstExpr()

This will simplify separating While/For from BlockStmt and adding new ASTs in the future

Compiled: STD clang/darwin, STD gcc/linux64, LLVM gcc/linux64
Tested: paratest -verify
Review: None