[Internalize] Preserve __stack_chk_fail in Internalizer correctly

Move the section collecting `AlwaysPreserved` up before any
`maybeInternalize` is called. Otherwise, functions in `AlwaysPreserved` (in this case, `__stack_chk_fail`)
are not preserved.

Reviewed By: MaskRay, tejohnson

Differential Revision: https://reviews.llvm.org/D112684