[PGO] Read VP raw data without depending on the Value field

Before this patch, each function's on-disk VP data is 'pointed'
to by the Value field of per-function ProfileData structue, and 
read relies on this field (relocated with ValueDataDelta field)
to read the value data. However this means the Value field needs
to be updated during runtime before dumping, which creates undesirable
data races.

With this patch, the reading of VP data no longer depends on Value
field. There is no format change. ValueDataDelta header field becomes
obsolute but will be kept for compatibility reason (will be removed
next time the raw format change is needed).

llvm-svn: 255329