[BOLT] Stop using std::iterator (NFC)

Without this patch, I get warnings like:

  bolt/include/bolt/Core/BinaryContext.h:108:19: error:
  'iterator<std::bidirectional_iterator_tag,
  llvm::bolt::BinarySection>' is deprecated
  [-Werror,-Wdeprecated-declarations]

This patch fixes those warnings by defining iterator_category,
value_type, etc.

This patch intentionally leaves duplicate types like FilterIterator::T
and FilterIterator::PointerT intact to avoid mixing the fix and the
cleanup.

Differential Revision: https://reviews.llvm.org/D133650