The ID can eg. de used in MCSymbol names to differentiate the ones
that need to be created for every unit.
The ID is a constructor parameter and not a static class member so
there is no issue with counter updates if we decide to thread that
code.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@232245 91177308-0d34-0410-b5e6-96231b3b80d8