[clang] Change ordering of PreableCallbacks to make sure PP can be referenced in them

Currently, BeforeExecute is called before BeginSourceFile which does not allow
using PP in the callbacks. Change the ordering to ensure it is possible.
This is a prerequisite for D114370.

Originated from a discussion with @kadircet.

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D114525