[XRay][compiler-rt] Profiling Mode: Include file header in buffers

Summary:
This change provides access to the file header even in the in-memory
buffer processing. This allows in-memory processing of the buffers to
also check the version, and the format, of the profile data.

Reviewers: eizan, kpw

Reviewed By: eizan

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D50037

llvm-svn: 338347