Internally store the body of a BlockExpr using a Stmt* instead of a...

Internally store the body of a BlockExpr using a Stmt* instead of a CompoundStmt*, and use the getBody() method to do the appropriate checking.  This both removes the type-punning warnings in Expr.cpp and also makes BlockExpr have more consistency checks against modifications to its body (via StmtIterator).

llvm-svn: 56710