[memprof] Deduplicate and outline frame storage in the memprof profile.

The current implementation of memprof information in the indexed profile
format stores the representation of each calling context fram inline.
This patch uses an interned representation where the frame contents are
stored in a separate on-disk hash table. The table is indexed via a hash
of the contents of the frame. With this patch, the compressed size of a
large memprof profile reduces by ~22%.

Reviewed By: tejohnson

Differential Revision: https://reviews.llvm.org/D123094