InstrProf: Simplify counting a file's regions when writing coverage (NFC)

When writing a coverage mapping we iterate through the mapping regions
in order of FileID, but we were then repeatedly searching from the
beginning of the list to count the number of regions with a given
FileID.

It is simpler and more efficient to search forward from the current
iterator to find the number of regions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218842 91177308-0d34-0410-b5e6-96231b3b80d8