Fix #10354 from Juan Alday. As he pointed out, the declaration (in detail/utf8_codecvt_facet.hpp) and definition (in detail/utf8_codecvt_facet.ipp) need to use the same signature.  Reapply Marshall Clow's [SVN r81616] and [SVN r81877] from November and December, 2012, as these appear to correctly supply const for the msvc/dinkumware library (since fixed) that used a do_length() first argument. Revert the portion of [SVN r86722] that removed BOOST_CODECVT_DO_LENGTH_CONST from the do_length() signature in utf8_codecvt_facet.ipp so that it stays in sync with utf8_codecvt_facet.hpp. For the same reason, copy the __IBMCPP__ workaround from utf8_codecvt_facet.ipp to utf8_codecvt_facet.hpp. Verified all libraries (filesystem, log, program_options, property_tree, serialization) that use utf8_codecvt_facet passed all tests before, and then after, these changes. Also applied minor edits to the use comments embedded in utf8_codecvt_facet.hpp.