[ARCMT][NFC] Reduce #include dependencies

Replace some #includes in ARCMigrate source files with more specific includes
and forward declarations. This reduces the number of files that need to be
rebuilt when a header changes (and saves like 1 second of build time). For
example, several files no longer need to be rebuilt when the list of static
analyzer checkers(!) changes.

Differential Revision: https://reviews.llvm.org/D74385