[MemProf] Defer profile file setup until dump time

With all of the writing of the memprof profile consolidated into one
place, there is no need to set up the profile file (which creates the
file and also redirects all printing from the runtime to it) until we
are ready to dump the profile.

This allows errors and other messages to be dumped to stderr instead of
the profile file, which by default is in a binary format. Additionally,
reset the output file to stderr after dumping the profile so that any
requested memprof allocator statistics are printed to stderr.

Differential Revision: https://reviews.llvm.org/D138175