Fixed comment in header of Block Frequency Impl and added text for C++ mode.

This is a generic block implementation that works on more than machine blocks.
The C++ mode addition is a bonus due to the extra space provided.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186175 91177308-0d34-0410-b5e6-96231b3b80d8