[InstrProf][NFC] Move function out of InstrProf.h

`createIRLevelProfileFlagVar()` seems to be only used in
`PGOInstrumentation.cpp` so we move it to that file. Then it can also
take advantage of directly using options rather than passing them as
arguments.

Reviewed By: kyulee, phosek

Differential Revision: https://reviews.llvm.org/D118097