InstrProf: Hide SourceMappingRegion's internals (NFC)

This struct has some members that are accessed directly and others
that need accessors, but it's all just public. This is confusing, so
I've changed it to a class and made more members private.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@218737 91177308-0d34-0410-b5e6-96231b3b80d8