GCC AutoFDO profile reader - Initial support.
This adds enough machinery to support reading simple GCC AutoFDO
profiles. It now supports reading flat profiles (no function calls).
Subsequent patches will add support for:
- Inlined calls (in particular, the inline call stack is not traversed to accumulate samples).
- Working sets and modules. These are used mostly for GCC's LIPO optimizations, so they're not needed in LLVM atm. I'm not sure that we will ever need them. For now, I've if0'd around the calls.
The patch also adds support in GCOV.h for gcov version V704 (generated
by GCC's profile conversion tool).