[NFC] Add raw_ostream parameter to printIR routines

This is a non-functional-change to generalize the printIR routines so that
the output can be saved and manipulated rather than being directly output
to dbgs(). This is a prerequisite change for many upcoming changes that
allow new ways of examining changes made to the IR in the new pass manager.

Reviewed By: aeubanks (Arthur Eubanks)

Differential Revision: https://reviews.llvm.org/D85999