[AST] Only store the needed data in WhileStmt

[AST] Only store the needed data in WhileStmt

Don't store the data for the condition variable if not needed.
This cuts the size of WhileStmt by up to a pointer.
The order of the children is kept the same.

Differential Revision: https://reviews.llvm.org/D53715

Reviewed By: rjmccall

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@345597 91177308-0d34-0410-b5e6-96231b3b80d8