[MBFIWrapper] Add a new function getBlockProfileCount

MBFIWrapper keeps track of block frequencies of newly created blocks and
modified blocks, modified block frequencies should also impact block profile
count. This class doesn't provide interface getBlockProfileCount, users can only
use the underlying MBFI to query profile count, the underlying MBFI doesn't know
the modifications made in MBFIWrapper, so it either provides stale profile count
for modified block or simply crashes on new blocks.

So this patch add function getBlockProfileCount to class MBFIWrapper to handle
new blocks or modified blocks.

Differential Revision: https://reviews.llvm.org/D87802