[MemProf] Clean up MemProf instrumentation pass invocation

First, removes the invocation of the memprof instrumentation passes from
the end of the module simplification pass builder, where it doesn't
really belong. However, it turns out that this was never being invoked,
as it is guarded by an internal option not used anywhere (even tests).

These passes are actually added via clang under the -fmemory-profile
option. Changed this to add via the EP callback interface, similar to
the sanitizer passes. They are added to the EP for the end of the
optimization pipeline, which is roughly where they were being added
already (end of the pre-LTO link pipelines and non-LTO optimization
pipeline).

Ideally we should plumb the output file through to LLVM and set it up
there, so I have added a TODO.

Differential Revision: https://reviews.llvm.org/D151593