[memprof] Fix frame deserialization on big endian systems.

We write the memprof internal call frame data in little endian format.
However when reading the frame information we were casting it directly
to a MemProfRecord::Frame pointer. In this change we add a separate
deserialization method which uses an endian reader to read the bytes as
little endian.

This fixes https://lab.llvm.org/buildbot/#/builders/100/builds/12940

Differential Revision: https://reviews.llvm.org/D120093