[PGO] Allow value profile writer interface to allocated target buffer

[PGO] Allow value profile writer interface to allocated target buffer

Raw profile writer needs to write all data of one kind in one continuous block,
so the buffer needs to be pre-allocated and passed to the writer method in
pieces for function profile data. The change adds the support for raw value data
writing.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@254219 91177308-0d34-0410-b5e6-96231b3b80d8