[clang] Add fixit for Wreorder-ctor

Create fix-it hints to fix the order of constructors.
To make this a lot simpler, I've grouped all the warnings for each out of order initializer into 1.
This is necessary as fixing one initializer would often interfere with other initializers.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D98745